SIXEL: cursor positioning and shared palette in TEs

[jerch]

(Partially offtopic for VT340, where the palette is always a globally shared one...)

I wonder if we manage to get a somewhat consistent behavior for TEs implementing sixels for these aspects:

• cursor positioning Imho the final cursor positioning after sixels is a wild-grown nightmare among TEs currently, where most modes makes not much sense or can be achieved by additional cursor movements. Thus I'd like to discuss, if we can simplify that to 2 basic modes, that are hard to achieve otherwise:
  • text cursor at VT340 position: Make this the default mode. Its big advantage is the fact, that it can dump sixels up to the last line without doing another scroll. Furthermore it places the cursor at image offset, which has a big advantage for aligned text output.
  • cursor at last image cell: I see this as the most natural cursor behavior in terms of cursor advance, if one sees sixel input as a block of cells. This is similar to what xterm and mintty do, but they go one cell further and the nightmare starts - what, if the cursor ends at COLS+1? Should the cursor wrap? Based on which setting? DECAWM? I'd say no, imho no wrapping should happen at all, instead place the text cursor in the last image cell. From there app can move the cursor. This mode has the advantage to automatically give information about the printout size in both image dimensions. If we dont allow any automatic wrapping, the flaw of xterm's mode with the last line scrolling also vanishes.
• private vs. shared palette handling This one is not really VT340 related, as the palette is always shared. What I'd like to discuss here, is the question, how newer TEs could possibly handle private and shared in a uniform way.
  • private palette I'd like to propose, that this mode always bootstraps the initial palette from some predefined palette, an app user can expect to work with without any own color definition (we currently use VT340-color for that, I am also fine to use ANSI256 if we can agree on that).
  • shared palette This mode is really tricky for most newer TEs, as many underlying sixel libs dont expose indexed images, but realize colors immediately (I call this printer mode here). I still hesitate to implement a true terminal-wide shared palette handling, but would do that, if there is demand. In the meantime I'd like to propose a less disruptive shared mode instead, which still works in printer mode with immediate coloring, but carries the created palette forward to the next incoming sixel image. This would allow to lower data pressure by omitting palette creation in consecutive images using color definitions from an earlier image.

I am not really determined to the modes as proposed above, but I think we should find an agreement to these aspects, so app devs have a model expected to work in a certain way.

[j4james]

Cursor positioning: I personally don't see a need for anything other than the standard behavior, and I'd be very impressed if we could even get TEs to agree on that. I'd just be happy to get XTerm following the standard, and after that I don't care. With any luck other TEs will fall in line like they did with DECSDM.

Palette handling: For this I'd like something that by default is as close to the VT340 as possible while still being usable with modern apps, ideally with a strict compatibility mode as well. I'm still not exactly sure how I'm going to achieve that, but I can't imagine other TEs wanting to take the same approach.

That said, instead of trying to get TEs to agree on exactly matching behavior, we could possibly come up with a minimal set of guarantees that apps could rely on. For example:

1. If you set a palette value in one image, it will be carried forward to the next image, but only if you assign colors in the order #1, #2, #3 (i.e. following VT340 rules).
2. If you output an image with a new set of palette values, it won't affect previous images you've output by default (i.e. outside of any strict compatibility modes the TE might have).
3. If you change palette values half way through an image being output, it may or may not effect the pixels in that image that have already been displayed (in other words, don't to that).

That doesn't seem like a lot to ask of TEs, but still provides a reasonable base for apps to work with.

[dankamongmen]

2. If you output an image with a new set of palette values, it won't affect previous images you've output by default (i.e. outside of any strict compatibility modes the TE might have).

it would be good to lock down how the palette size relates to independent palettes. i.e. if it's 16, can i have 2 images with 16 distinct colors each? or is it simply two palette "namespaces" backed by a hard 16 colors?

[j4james]

it would be good to lock down how the palette size relates to independent palettes. i.e. if it's 16, can i have 2 images with 16 distinct colors each? or is it simply two palette "namespaces" backed by a hard 16 colors?

On modern terminals you'll have access to N distinct colors in each image, where N is typically 256 or more. On DEC terminals, and strict emulators, you'll have a hard limit of 16 colors or less shared across all images.

I'm not sure what you mean by namespaces, but modern terminals don't support color mapping in the sense that DEC terminals do. On modern terminals, sixel color #0 will usually always map to palette entry 0. On DEC terminals (and strict emulators), the first color defined will update palette entry 1 (not 0), regardless of the color number, so different images may have different color number mappings. But this is only really of relevance to palette inheritance, and as long as you assign colors in the order I recommended, it shouldn't matter.

[dankamongmen]

oh of course; i was just emphasizing that this be addressed in any protostandard y'all're developing

[jerch]

Cursor positioning: I personally don't see a need for anything other than the standard behavior, and I'd be very impressed if we could even get TEs to agree on that. I'd just be happy to get XTerm following the standard, and after that I don't care. With any luck other TEs will fall in line like they did with DECSDM.

Having less modes for cursor positioning will help app side to get things right causing less fuzz/frictions, thats true. If the xterm mode gets not removed, it needs a revamp due to its n+1 cell + scrolling issue. Imho there is some value in that mode for app side if the cursor stops at n, because it can signal the width of a sixel image with an additional CPR, which is not possible with vt340 mode alone (e.g. without inspecting the image itself). So my second suggestion is a compromise to still offer that benefit, but without the scrolling flaw. From a technical viewpoint I dont care much, whether we have one or 5+ cursor modes (they are really easy to implement on TE side), but for the sake of usability from app side it should not be more than one or two. Currently my addon implements the xterm/mintty mode, but I will change that to vt340 as default. On a technical sidenote - the mintty offset wrapping can be "emulated" with vt340 mode + LF, the xterm mode with my second suggestion + a conditional CRLF or CUF.

If you set a palette value in one image, it will be carried forward to the next image, but only if you assign colors in the order #1, #2, #3 (i.e. following VT340 rules).

Yes I second that, it is basically what I meant with "palette sharing" - colors are carried over.

If you output an image with a new set of palette values, it won't affect previous images you've output by default (i.e. outside of any strict compatibility modes the TE might have).

I agree, again images are basically decoupled in their palette color values (beside some initial palette values from previous image).

If you change palette values half way through an image being output, it may or may not effect the pixels in that image that have already been displayed (in other words, don't to that).

Well, libsixel promotes that as "high-color" - switching the palette within an image. It only works in printer mode, thus is kinda a misuse of an implementation detail. mlterm supports that (prolly all libsixel based TEs), xterm does not (always works with indexed intermediate images, where a color slot change would re-color previous pixels of that image). Imho this should not be promoted, as it is not covered by any spec notion and cannot be relied on. (Mind you - it theoretically allows to apply 1M RGB colors with a palette size of one by just changing that single slot color over and over.)

What I take from this - we dont even need a private palette option, that would always start from a fixed palette, e.g. ground the palette back to vt340 colors or any other predefined one? I am still unsure about this, as it gives app side some convenience about the colors to be expected. How would one restore the default palette on a VT340 before start drawing?

[hackerb9]

What I take from this - we dont even need a private palette option, that would always start from a fixed palette, e.g. ground the palette back to vt340 colors or any other predefined one? I am still unsure about this, as it gives app side some convenience about the colors to be expected. How would one restore the default palette on a VT340 before start drawing?

Restoring the palette on the VT340 before sending sixels can be done in several different ways. Here are the ones I've seen so far:

0. Do nothing. That's not as ridiculous as it sounds since one can simply use #7 as the foreground and #0 as the background and know that it will match the text colours.
1. Check if the colour table is correct using DECRQTSR (Request Terminal State Report)
2. Set the colour table to known values using DECRSTS (Reset Terminal State)
3. Change the colours using ReGIS (untested)
4. Switch between the two colormaps using DECSTGLT (Set Text / Graphics Look Up Table) [buggy?]
5. Send a sixel file that simply sets the palette and quits. .

There may be others I'm forgetting.

[j4james]

it can signal the width of a sixel image with an additional CPR, which is not possible with vt340 mode alone (e.g. without inspecting the image itself).

If you're an app that actually needs this functionality, which of these options would you honestly choose?

1. Inspecting the image itself, which is guaranteed to work.
2. Using a mode that half the terminals don't support, and those that do don't even implement in the same way, and which also requires an additional CPR query.
3. Yet another mode still to be invented, that'll likely have most of the same problems as 2, but possibly with fewer terminals implementing it.

I really can't imagine this being widely used.

Well, libsixel promotes that as "high-color" - switching the palette within an image. It only works in printer mode, thus is kinda a misuse of an implementation detail.

This is an interesting case, but it adds so much to the file size which I just don't think is justified for typical image viewing in the terminal. It might be something I'd consider as a mode one day if there was a huge demand for it, but I doubt that.

For personal use, I prefer having the ability to do palette animation on the image, even if it only works in my terminal.

How would one restore the default palette on a VT340 before start drawing?

There's always RIS, which most terminals support, but obviously not a great choice for apps to use. I've also considered resetting the palette with DECSTR, since that's what the VT240 does (and a couple of other modern terminals). But even with wider support I wouldn't recommend that to apps, because it can have some weird effects on some terminals. And anyway there's no guarantee what you'll get - different TEs have different defaults.

Again I don't imagine apps relying on something like this just to save a couple of bytes.

[hackerb9]

DECSTR was certainly what I expected would reset the palette before I actually tested it on a VT340. I wish I knew why it doesn't work. Was that a mistake? If it worked on the VT240, then perhaps Digital had a good reason.

On the other hand, DEC's ANSI Compatible Printer Protocol Programming Manual v2 (EK-PPLV2-PM) from 1994 mentions that DECSTR does reset the palette, albeit to all black for mono-printer compatibility.

Color numbers remain assigned even after leaving and reentering Sixel
Graphics mode. At power-up, Soft Terminal Reset (DECSTR), Select
Conformance Level (DECSCL), and Reset to Initial State (RIS), the
device assigns all color numbers to black.

Upon entering Sixel Graphics mode, the device selects color 0.

@jerch writes:

I'd like to propose, that this mode always bootstraps the initial palette from some predefined palette, an app user can expect to work with without any own color definition (we currently use VT340-color for that, I am also fine to use ANSI256 if we can agree on that).

Have you considered using something like the RGB colour map table from EK-PPLV2-PM? In the chapter on sixels, it describes the equation for converting from an index number to RGB values. It is designed so that one can create a sixel image that looks the same on devices with different colour capabilities, but perhaps could be repurposed or modified as a default colour map.

Here is what it says:

RGB Color Map

Table 5-5 shows the colors available for 8-color and 64-color devices. To
transform the offset found in Table 5-5 into its RGB components, apply the
following algorithm to the offset:

C = Offset
B₁ = C mod 4
G₁ = (C - B₁) mod 16
R₁ = (C - B₁ - G₁) mod 64

B = 25 * B₁
G = 25 * G₁/4
R = 25 * R₁/16

Example:

C = Color 41 — Khaki

B₁ = 41 mod 4 = 1
G₁ = 40 mod 16 = 8
R₁ = 32 mod 64 = 32

B = 25 * 1 = 25 G = 25 * 8 / 4 = 50
R = 25 * 32 / 16 = 50

RGB parameters range from 0–100.
x mod y is the remainder when x is divided by y.

Note

If you are writing an 8-color application, use the offsets for red, green,
blue, yellow, magenta, and cyan from the 64-color table. Then, if
your application is used on a 64-color device, the colors will still be as
expected.

[hackerb9]

If by ANSI256 you mean the colours XTerm uses, I think that that would be an acceptable default palette. I'd prefer it if the first sixteen colours were identical to the VT340, but ANSI256 does have some nice properties:

• Very well known
• Has the most important colours in the right palette position for a VT340:
  • #0 (background) is black
  • #7 (text foreground) is light grey
  • #15 (bold text foreground) is white
• Connection with text colours means a terminal emulator may choose to have the sixel palette change when text colours are redefined.

And a few downsides:

• Sixteen colour images for the VT340 would not work on newer terminals nor vice versa.
• Having the colours in the right position isn't actually helpful: it only matters if someone sends the palette to a VT340 and messes up the text colours, but we're talking about the default for when no palette is sent.
• Colour #7 is actually 75% grey, which is too bright for the VT340's 46% grey text.
• Similarly, #15 is 100% white which is too bright for the VT340 which uses 79% grey. (#C9C9C9)
• Changing the sixel colours when changing text colours may have benefits, but the biggest benefit of that connection on the VT340 would not be achieved: being able to create a sixel image that uses the current foreground and background colours, no matter what they are.

Another idea for a default palette

Just spitballing here, but what if the first sixteen colours were defined the same as a VT340, including the special connection with the text colours? Yes, if a user changes the default foreground and background colours, then the palette would be different, but that's actually the way it works on the VT340. One can change those colours in the Set-Up menu and it affects the default palette.

I'm imagining it being bidirectional: setting the text colour changes palette entries #7 and #0, and setting those two entries changes the current text colour. But that's probably not acceptable. It may be possible to make the connection go one way: defining colours in a sixel image affects that current image's palette but would not change the text colour. This might be a problem as it means the palette changes for those two entries would get reset to the text colours when the next sixel image is displayed.

[hackerb9]

cursor at last image cell: I see this as the most natural cursor behavior in terms of cursor advance, if one sees sixel input as a block of cells. This is similar to what xterm and mintty do, but they go one cell further and the nightmare starts - what, if the cursor ends at COLS+1? Should the cursor wrap? Based on which setting? DECAWM? I'd say no, imho no wrapping should happen at all, instead place the text cursor in the last image cell. From there app can move the cursor. This mode has the advantage to automatically give information about the printout size in both image dimensions. If we dont allow any automatic wrapping, the flaw of xterm's mode with the last line scrolling also vanishes.

A small note: the final cursor position does not give information about the size in both dimensions. I think that would have been a good idea, but on the VT340 the text cursor only changes in row, not column. I believe the manual described it as an implicit Graphics Carriage Return ($).

One benefit of this is we do not need to worry about what happens at COLS + 1.

UPDATE: Here is the entry from EK-PLv2 PM, p. 5-10:

5.6.3 State After Exiting Sixel Graphics Mode

After exiting Sixel Graphics mode, the device returns to text mode with the following conditions:

• The text active horizontal position is the same as just before entering Sixel Graphics mode.
• The text active vertical position is the same as the graphics active vertical position. As a result, the baseline of subsequent text is contiguous with a point 700 centipoints below the top pixel of the last sixel line. In addition, the first text mode vertical motion command (LF, VT, and so on) causes the device to advance to the next text mode line (get back on grid) before executing the vertical motion command.

[jerch]

@j4james

For personal use, I prefer having the ability to do palette animation on the image, even if it only works in my terminal.

Yes thats straight up impossible with immediate coloring as in printer mode. My personal stance here - I'd favor higher output quality over palette animation (I see absolutely no need for the latter). I still dont like the high-color feature as such, because it is not covered by official docs. I kinda accidentally implemented the sixel decoder to work with that feature correctly, but it certainly undermines the original palette idea on old terminals. To me this is not more than an unreliable hack done by libsixel.

about RIS and DECSTR: I coupled palette reset to both. For RIS is was quite clear to me, but not so for DECSTR. In the end I also went with DECSTR here, because I also purge the image storage (basically resetting whole image handling back to initial values). If there are good reasons not to do this for DECSTR, I would change that. Any clue why DEC went here different ways for VT240 and VT340 (and printers)?

@hackerb9 Thx for the list of possible palette reset commands. But dont they all need to know app side the palette in advance (prolly beside RIS)? So there is no single command in the sense of "load default palette" without any other side effects, right?

Yes with "ANSI256" I mean xterm's default 256-indexed colors. (The term ANSI256 is technically wrong, as there was no 256-color space from ANSI at all, but peeps made that up from the lower 8/16 colors called ANSI colors.) The fact that this palette is not VT340 compatible is really nasty (and the reason why I switched back to vt340 colors). The effect gets obvious on those old sixel images from the 90s, that dont define own colors (they look quite different with ANSI256).

Just spitballing here, but what if the first sixteen colours were defined the same as a VT340, including the special connection with the text colours? Yes, if a user changes the default foreground and background colours, then the palette would be different, but that's actually the way it works on the VT340. One can change those colours in the Set-Up menu and it affects the default palette.

Thats an interesting idea. Problem I see here - the lower 16 positions in ANSI256 are exactly that on xterm, but xterm simply went with different default low colors in the first place, no clue why. This cannot be backmerged with VT340 colors without frictions, a proper backmerge would mean to also adopt VT340 ANSI color values for text (partially giving up xterm colors). Or a halfway backmerge to went with VT340 colors only in sixel default palette, thus losing the text-image color inter-linkage. I dont have an answer for that atm - what I currently do: use 16 VT340 colors as default, with no relation to any text color (no inter-linkage at all, both may mutate independently).

I really like the inter-linkage idea, as it would give the possibility to more uniform (coupled) coloring, as it would have been on a VT340, but dont know how to bridge the xterm text palette vs VT340 palette differences.

@j4james, @hackerb9 About final cursor state: Yes I am aware of that handling (still might not understand all details about it).

In terms of the sixel protocol mechanics and how low-level it is, DEC's behavior it the only one that makes sense:

• sixel started somewhere (aka initial offset)
• right extend is not formally specified, may be ragged by individual 6-pixel line progression to the right side
• vertical "print head" position is fixed by amount of GLFs (no backmove possible), thus can be mapped back properly at the end of the image --> final ROW value
• What to choose for final COL value? Right side ragged --> no proper value can be derived, just go with initial COL value (offset)

In that sense the xterm mode is completely flawned, as the spec does not define a "right border" (not even sure how xterm deals with the right ragged problem here). Sixel is just a "print on top" protocol with very limited guarantees of a left border and the overall vertical progression. On the other side DEC STD 070 tells us to use proper raster attributes ideally with xy extends, and that a conforming encoder may not run beyond these limits. So DEC saw a need for formalizing a rectangular image area, but prolly could only half adopt it through their older already existing lvl1 devices (therefore stating, that a decoder shall not limit outputs by xy extends in raster attributes). Idk the real issues behind that lvl1 vs lvl2 thingy, thus might jump to wrong conclusions here.

So to summarize this for myself - I think xterm mode should be removed completely, as even my compromise to keep it alive without the COLS+1 flaw has that right side ragged issue. In a strict sense it is not possible to derive a right border for sixels.

[j4james]

Just spitballing here, but what if the first sixteen colours were defined the same as a VT340, including the special connection with the text colours? Yes, if a user changes the default foreground and background colours, then the palette would be different, but that's actually the way it works on the VT340. One can change those colours in the Set-Up menu and it affects the default palette.

@hackerb9 This sounds like what I was planning for my "strict" compatibility mode. My intention was the text palette and the graphics palette would be identical, and any changes to one would affect the other. Since the text palette is controlled by the user's color scheme choice, that would apply to the graphics palette as well. If a user wanted to match the default VT340 palette exactly, they would just need to select an appropriate color scheme (I would probably argue to have a VT340 scheme included in the app by default).

For the "modern" usage, though, the text and graphics palettes have got to be separate, so there's no need for them to be the same. And since a lot of modern terminals default the first 16 colors to match the VT340 (or at least something quite similar), that's almost a de-facto standard at this point. After the first 16 there is more variation, but using the XTerm 6x6x6 + grayscale palette is a common choice (although XTerm itself just uses black for this range). And after 256, it'll typically be some shade of black/white/gray.

My personal stance here - I'd favor higher output quality over palette animation

@jerch I totally understand that. I reckon at least half of modern terminals work that way, so it's not an unreasonable choice. But my preference is for my defaults to be as close to VT340 behavior as possible, as long as that doesn't break any widely used modern apps, and there are so many fun things you can do with palette animation, so I would hate to lose that functionality.

In that sense the xterm mode is completely flawned, as the spec does not define a "right border" (not even sure how xterm deals with the right ragged problem here)

Note that the right border mode is an RLogin invention (assuming you're talking about 8452). XTerm just copied that, but not correctly, so they don't exactly match. XTerm's default is to place the cursor in the leftmost column of the screen, which obviously isn't correct either. I'm still hoping to convince XTerm to fix this at some point if I ever get around to writing a patch for it.

[jerch]

@j4james

And since a lot of modern terminals default the first 16 colors to match the VT340 (or at least something quite similar), that's almost a de-facto standard at this point. After the first 16 there is more variation, but using the XTerm 6x6x6 + grayscale palette is a common choice (although XTerm itself just uses black for this range).

If we can agree on that, I'd do it likewise - lower 16 colors from VT340 + filling up to 256 from xterm's 256-indexed colors.

But my preference is for my defaults to be as close to VT340 behavior as possible, as long as that doesn't break any widely used modern apps, and there are so many fun things you can do with palette animation, so I would hate to lose that functionality.

Yeah, and for proper VT340 emulation it is even mandatory that way. I'm still unsure what to make of that immediate vs always indexed coloring issue (printer vs. terminal mode), I def. like the higher quality possibilities by redefining the palette on the fly, but hate it for being off the record. libsixel kinda let it out of the bottle, hard to put it back there. I only see two ways for this, either discourage its usage completely, or try to formalize it somehow (as extension on XTSMGRAPHICS?), so app side can use it conditionally, if the TE indicates support.

Note that the right border mode is an RLogin invention (assuming you're talking about 8452). XTerm just copied that, but not correctly, so they don't exactly match.

Oops, thx for clarification. My association with xterm here was not meant to blame anyone, it just occurred first to me from xterm settings (did not know about RLogin). Well, I gonna remove these cursor xterm/Rlogin/mintty modes for now, as they dont fit the basic sixel idea (RLogin) or can easily be emulated from VT340 behavior (xterm+mintty). The right border idea is nice for strictly rectangular images, but thats unreliable for sixels.

[jerch]

My take-aways from this thread:

• remove all foreign cursor modes, just stick with VT340 mode for DECSDM=off
• remove private/shared palette separation, palette entries are always carried forward, if not reset by other means in between

Thx for the discussion, helped me to get some things straight.

One more thing that still bugs me - there is no easy way to tell the TE to reset to the initial palette (yes RIS and DECSTR can do that be meh, with heavy side effects). So what would you say about this idea - whenever app side requests palette size change from XTSMGRAPHICS, it also would reset to the initial palette? This could be made as a guarantee, so even if the size does not change, at least the palette would get refreshed. I know thats like piggybacking it, but a "naked" palette resize leads to weird undefined state, so it came to me to couple it with a palette reset.

[dankamongmen]

One more thing that still bugs me - there is no easy way to tell the TE to reset to the initial palette (yes RIS and DECSTR can do that be meh, with heavy side effects). So what would you say about this idea - whenever app side requests palette size change from XTSMGRAPHICS, it also would reset to the initial palette? This could be made as a guarantee, so even if the size does not change, at least the palette would get refreshed. I know thats like piggybacking it, but a "naked" palette size change leads to weird undefined state, so it came to me to couple it with a palette reset.

you might want to tie this in with XT*COLORS https://sw.kovidgoyal.net/kitty/color-stack/

[jerch]

@dankamongmen Thx for the suggestion. I cannot use these, as we prolly never will implement them (they are really bad in terms of terminal state or app side assumptions, see - https://github.com/xtermjs/xterm.js/issues/2570).

[j4james]

So what would you say about this idea - whenever app side requests palette size change from XTSMGRAPHICS, it also would reset to the initial palette?

I wouldn't say don't do it, because I don't really care, but I'm going to do my best to avoid implementing XTSMGRAPHICS if at all possible, so I'd be unlikely to support this.

For the same reason, if I were ever to support the sixel high-color mode we were discussing, I would probably argue for a simple SM/RM mode to enable the functionality, rather than adding more crud to XTSMGRAPHICS. Support for the mode could then be detected with DECRQM.

[dankamongmen]

@dankamongmen Thx for the suggestion. I cannot use these, as we prolly never will implement them (they are really bad in terms of terminal state or app side assumptions, see - https://github.com/xtermjs/xterm.js/issues/2570).

interesting. regarding your comment

This means they cannot simply exit early anymore (in case of errors), now they have to to cleanup the terminal state as well in an exception or exit handler.

this does indeed seem a fundamental requirement for a number of reasons? Notcurses sets up handlers for default termination signals (unless a flag is provided) for exactly this reason.

[jerch]

@j4james Yes true, XTSMGRAPHICS is somewhat a lame excuse for our issues with DECSET/SM. I currently try to stay away from any additions to DECSET/SM realms, as we have no clean story yet, how to do that. I hate the wild-growth there by other TEs doing as they please.

@dankamongmen

this does indeed seem a fundamental requirement for a number of reasons? Notcurses sets up handlers for default termination signals (unless a flag is provided) for exactly this reason.

Sure if you control the terminal all the time as a the only app, and take care to clean up that stack - it works. But if I remember correctly, the other commenter wanted that for a component compiler thingy with multiple processes. There you prolly cannot control that state anymore (because of processes dying, exiting ungracefully etc) screwing up the colors stack in the terminal sooner or later. It was something along these lines, cant remember all the details anymore.

[j4james]

I hate the wild-growth there by other TEs doing as they please.

I've very much in agreement with you here, so I'm not suggesting rushing into anything. But if there's a compelling use case for this functionality, and you can get a couple of other terminals to agree on a mode for it, I don't think that would be unreasonable. And until then you could always just offer it as a user-controlled config option.

And coming back to the usage of DECSTR for palette reset, have you tried that on your VT525? I'm assuming you can set color table values with DECRSTS, so would those changes then be reset by a DECSTR? If yes, then I'd have no qualms about using DECSTR to reset the graphics palette by default too (maybe just disabling that functionality when in strict VT340 compatibility mode).

[jerch]

And coming back to the usage of DECSTR for palette reset, have you tried that on your VT525? I'm assuming you can set color table values with DECRSTS, so would those changes then be reset by a DECSTR?

Nope, DECSTR does not effect any colors on the VT525. But I have a hard time to understand the color sequences, and prolly will not get down to it before feeling comfortable with the env settings. Any clue, what Pc - color number 0-255 might be in DECRSTS?

As a quick starter, at least the following I have figured - In VT525 mode colors can either be:

• ANSI colors: palette is 16 colors with typical darker (0-7) and brighter variants (8-15), very similar to xterm, SGR modes are fixed
• Alternate colors: palette looks similar to VT340 one, allows fine-grained colors settings over 16 SGR modes (bright, inverse ...)
• Monochrome: palette contains several grey shades (prolly mimicking VT240..VT510)
• more color modes for other terminal emulations (ASCII, Wyse ...)

The ANSI vs Alternate color palette distinction actually raises the question, if mixing VT340 color + filling up from xterm colors is the right choice for a default sixel palette on a more ANSI-like TE. How is that done on a VT340? Does it offer the ANSI palette by some setting?

[hackerb9]

Nope. VT340 never does ANSI colours. There is only one foreground colour (I'm fudging a little here), so it is (as far as I can tell) impossible to show text in multiple colours.

Fudge: bold text can be changed to another colour. I think dim might as well, but not underline. (Unlike the VT525?)

[Also, my VT340+ has an interesting feature when switching between colour maps 1 and 2 where the previous palette and the new palette are displayed simultaneously. I need to experiment with this more, but it may be the reason there was a rumour that the VT340+ could do show twice as many colours. It's possible the earlier VT340s (sans plus) did not have this quirk].

Still, even with all the fudging, the answer is, sorry, no ANSI.

[j4james]

Nope, DECSTR does not effect any colors on the VT525.

OK. Thanks for confirming that. I think then I'll probably not reset the palette in DECSTR by default either - only when specifically emulating the VT240. This seems fair enough, given that the VT240 was the only device that actually documented DECSTR as resetting the color table (at least as far as I could tell).

I wonder if perhaps they thought it wasn't necessary in later devices once you could save and restore the palette programmatically with DECRQTSR and DECRSTS.

Any clue, what Pc - color number 0-255 might be in DECRSTS?

I believe that's just the position in the palette that you want to change. For example, I'd expect to set color 7 (typically the text foreground) to bright magenta with something like this:

printf "\eP2$p7;2;100;0;100\e\\"

I wouldn't expect anything over 15 to work, unless those values are just modded with 16. And I'm assuming the numbers correspond directly with the SGR colors (i.e. color 0 is SGR 30, color 1 is SGR 31, and colors 8 and above are the bold equivalents).

Alternate colors: palette looks similar to VT340 one, allows fine-grained colors settings over 16 SGR modes (bright, inverse ...)

FYI, I think the alternate color mode(s) can also be enabled programmatically with DECSTGLT, and the color mappings can be set with DECATC and queried with DECRQSS. Once you're in a position to do some testing on your VT525 I'm very keen to learn exactly how these things work, because I think this architecture should be perfect for emulating all the different ways in which DEC terminals handled SGR attributes.

The ANSI vs Alternate color palette distinction actually raises the question, if mixing VT340 color + filling up from xterm colors is the right choice for a default sixel palette on a more ANSI-like TE.

Yeah, the more I think about this, the more I think you can't mix ANSI colors with Sixel, because the order is all wrong. If you've got a shared text-graphics palette, and you're updating palette entry 1 with DECRSTS, I'd expect that to change the blue color in Sixel images, but the SGR red color for text.

I suppose the simple answer is that you just don't support ANSI SGR sequences when you've got a "shared" text-graphics palette. The only color numbers that matter then are 0 for the background, 7 for the foreground, and 15 for bold. But my idea of having the user's chosen color scheme controlling the Sixel palette isn't really going to work. I'll need to rethink that.

Also, my VT340+ has an interesting feature when switching between colour maps 1 and 2 where the previous palette and the new palette are displayed simultaneously.

@hackerb9 I actually had an idea about that recently. I'll follow up in issue #12 where we were discussing this before.

[hackerb9]

One more thing that still bugs me - there is no easy way to tell the TE to reset to the initial palette (yes RIS and DECSTR can do that be meh, with heavy side effects). So what would you say about this idea - whenever app side requests palette size change from XTSMGRAPHICS, it also would reset to the initial palette? This could be made as a guarantee, so even if the size does not change, at least the palette would get refreshed. I know thats like piggybacking it, but a "naked" palette resize leads to weird undefined state, so it came to me to couple it with a palette reset.

I think resetting the colour palette when it resizes would not be wrong for a terminal to implement, but it shouldn't be a guarantee that programmers rely on. I feel similarly about DECSTR, but it is more complicated.

Although the VT340 does not support it, it feels like DEC made a mistake by omitting palette resetting from DECSTR. Of course, I've been very wrong in the past, so take my hunch with a grain of salt, but I am having trouble seeing any good reason (beyond VT340 compatibility) for future terminals to not interpret DECSTR in that way. As I mentioned, the sixel v2 printer specification says that DECSTR will reset the palette to a useful default (all black for printers). Why shouldn't a terminal act similarly?

I am not so sure that application programmers should consider it a "guarantee" of how terminals work, though. On the one hand, it seems that a way to say “Load default palette” is needed and that DECSTR is a natural way to do it.

On the other, it breaks VT340 compatibility and it is warping an old sequence for a new use, which I consider way worse than creating a new sequence that would simply be ignored by older terminals.

Fundamentally, DEC was not — at least, not originally — thinking about sixels as being device independent. Particularly in the 070 document, it is clear they expect one to know particulars of the sixel rendering device in order to generate a bitmap. That was what ultimately led me to proposing the XTSMGRAPHICS extension which allows one to query basic device information.

And it is that same lack of device independence now that leads me to think that if we must have a simple way to reset a terminal to its default palette, it ought to be a new sequence designed for the purpose. While terminal emulators could overload DECSTR, programmers would have no way of knowing if sending it would work or not and better programmers would avoid using it.

---

But, must we have a way to reset a terminal to its default palette? It may already be enough to reset to a known palette or to save a palette and restore it later using DECRQTSR (Request Terminal State Report) and DECRSTS (Reset Terminal State). Maybe I just lack imagination, but it seems like there are only two primary uses for resetting the palette and in both cases, the existing tools are actually better:

1. Quickly reset to a known palette so that sixel data that follows does not need to define colours.
2. Reset to the previous palette before a program was run.

• For 1, yes it is faster, but how much so? And is the palette actually known? It would be safer to just encode the desired palette in a program. I admit having a single sequence would be a large data savings which means much swifter sixel rendering, but bandwidth is mostly a problem for antiques like my VT340. If we were talking about a palette with hundreds of thousands of entries, that would be a different story, but we are only considering palettes with up to 256 colours. [Side note: while that seems reasonable given current bandwidth considerations, I see no natural reason that sixels would be limited to 256 colours. That's one of the reasons I was thinking about an algorithm for generating a colour palette, above.]

• For 2, I would think a user would prefer their colour palette set back to the way it was before the program was run, which is not necessarily the same as it was when the terminal is first powered on. Maybe they have login files that set the text colours to be more pleasing to them. Arbitrarily resetting the palette to system defaults after exiting would be seen as rude.

Any way, these are just my thoughts and I could easily be wrong.

[j4james]

Maybe I just lack imagination, but it seems like there are only two primary uses for resetting the palette and in both cases, the existing tools are actually better:

Yeah. This was my thinking as well.

I see no natural reason that sixels would be limited to 256 colours.

The only downside I've found with supporting more than 256 colors is that is makes it more difficult to support palette animation, but I doubt anyone other than me cares about that. I think most (although not all) modern terminals support at least 1024 colors, I suspect because that's a requirement for running Jexer.

[jerch]

Well, 256 is somewhat demanded by STD 070, not sure why they put that particular number there, it reads a bit like they tried to spec it for a broader application audience to have numbers to rely on (same with raster attribs). Prolly indexed-256 was state of the art these days. Technically we could extend the sixel mechanics to any palette size, with some restrictions on encoder side (some quantization algos can only operate in 2^n leading to 64, 128, 256, ... colors). I limited this to 4096 entries mainly to save memory, but its a compile time switch that easily could go with the full sixel RGB space (101³) wasting ~4MB for palette addressing. 4096 entries gives 12bit color depth, which is reasonably well for most images (except color gradients, which still can be fixed to some degree with proper dithering). And much higher palette sizes really suck on the bandwidth.

I think I will ditch the "reset to default palette somehow" idea for now, it was mainly a concern popping up from those older sixels, that did not define the colors. They def. would also show funny colors on VT340 after other sixels, so it can be seen as sloppy "sixel programming". I have not come across that issue from newer encoders, they always announce the colors upfront.