search

Plant-Tracer / webapp

Client and Server for web-based JavaScript app
GNU Affero General Public License v3.0
0 stars 2 forks source link

coordinate system mismatch: line graphs y axis flipped #601

Open sbarber2 opened 4 days ago

sbarber2 commented 4 days ago

So, it turns out that in the x, y vs time line graphs that the y axis values go down as the marked points move up in physical space, and vice versa.

This happens because in video frames the origin point (0,0) is the upper left corner of the frame.

However, it is extremely likely that the vast, vast majority of botany-oriented users of these videos are going to expect that (0,0) is the lower left corner of the frame. For example, Eric and JoAnn were very surprised to see the y values being graphed in the opposite direction of the way the plant was moving in physical space, where we think of height values increasing as we go, well, higher.

It's easy enough to flip the direction of the graph's y-axis. That is, we multiply y * -1 before we feed the data to the graph.

That said, does this (literal) difference in perspective mean that we should also be changing how the tracked data values are communicated to the user, for example in the CSV download files. That is, is it OK for the y pixel values to be going down while the y mm values are going up? Should we report the pixel values as if the frame origin were lower-left, as well? What's the desired result here?

sbarber2 commented 4 days ago

Heh, I can't tell yet if we're fixing a bug or changing how features work with this Issue.

Basically, this happened because we had a loose specification and no tests.

simsong commented 4 days ago

We call this a “flipped coordinate system.” In general, computers use flipped coordinate systems for screens and printouts. However, they are confusing for mathematical applications, so we should never use them in our application.

On Oct 18, 2024, at 11:41 AM, Steve Barber @.***> wrote:

So, it turns out that in the x, y vs time line graphs that the y axis values go down as the marked points move up in physical space, and vice versa.

This happens because in video frames the origin point (0,0) is the upper left corner of the frame.

However, it is extremely likely that the vast, vast majority of botany-oriented users of these videos are going to expect that (0,0) is the lower left corner of the frame. For example, Eric and JoAnn were very surprised to see the y values being graphed in the opposite direction of the way the plant was moving in physical space, where we think of height values increasing as we go, well, higher.

It's easy enough to flip the direction of the graph's y-axis. That is, we multiply y * -1 before we feed the data to the graph.

That said, does this (literal) difference in perspective mean that we should also be changing how the tracked data values are communicated to the user, for example in the CSV download files. That is, is it OK for the y pixel values to be going down while the y mm values are going up? Should we report the pixel values as if the frame origin were lower-left, as well? What's the desired result here?

— Reply to this email directly, view it on GitHub https://github.com/Plant-Tracer/webapp/issues/601, or unsubscribe https://github.com/notifications/unsubscribe-auth/AAMFHLGA66KUBDLRB4P745LZ4ET4HAVCNFSM6AAAAABQGG56IGVHI2DSMVQWIX3LMV43ASLTON2WKOZSGU4TOOBVGY3DAMI. You are receiving this because you are subscribed to this thread.

sbarber2 commented 3 days ago

we should never use [flipped coordinate systems] in our application.

And yet we have been and we do:

So, I think we have to decide about how and where to translate between the flipped movie frame coordinate system (because all the graphics and video libraries use that system), and the non-flipped user presentation coordinate system.

simsong commented 3 days ago

Well, it is fine to just flip them when you show them to the user. You just have to be careful about it.

Back on my NeXT machine, it was a system property, whether a View was flipped or not. It still is…

https://developer.apple.com/documentation/appkit/nsview/1483532-isflipped

https://developer.apple.com/documentation/appkit/nsgraphicscontext/1531568-isflipped

On Sat, Oct 19, 2024 at 9:48 AM Steve Barber @.***> wrote:

we should never use [flipped coordinate systems] in our application.

And yet we have been and we do:

  • Marker positions are stored and displayed to the user as flipped coordinates
  • Tracepoint x,y values are saved to the DB as flipped coordinates
  • The "Download CSV" csv files contain flipped coordinates
  • The 2 x,y vs t line graphs use and display flipped coordinates

So, I think we have to decide about how and where to translate between the flipped movie frame coordinate system (because all the graphics and video libraries use that system), and the non-flipped user presentation coordinate system.

— Reply to this email directly, view it on GitHub https://github.com/Plant-Tracer/webapp/issues/601#issuecomment-2423875099, or unsubscribe https://github.com/notifications/unsubscribe-auth/AAMFHLHNUQ34H6V2L45OMKLZ4JPIFAVCNFSM6AAAAABQGG56IGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDIMRTHA3TKMBZHE . You are receiving this because you commented.Message ID: @.***>