Variable width font support?

[stevesims]

Building on #120 it is possible using vdp-gl to use variable width fonts. We could, therefore, potentially allow their use inside agon-vdp.

NB this would be an advanced feature, and may not be entirely in keeping with agon-vdp functionality. Since the text cursor system in agon-vdp works on a character position, rather than a screen position, and thus relies on fixed width fonts adding this would be difficult and problematic. Were it added, it may need to use a different text cursor system.

[lennart-benschop]

BBC/Agon VDU does support the graphics cursor to write text (VDU 5(, so this might be adaptable for use with variable width fonts.

[stevesims]

yeah - nice idea. so a possibility here could be that we only support a variable width font via the graphics cursor

the problematic parts are areas that use character-based positioning. for instance VDU 31, which equates to BASIC's TAB statement, VDU 28 that sets a text viewport, VDU 23,0,&82 that sends the cursor position back to MOS, and VDU 23,0,&83,x;y; which reads the character on-screen at the given text position

(it's curious that VDU 23,0,&83 is using 16-bit values for the text position - that feels like a mistake. perhaps the VDP should adjust the behaviour of this command when VDU 5 has been set to instead accept screen coordinates?)

[stevesims]

currently the underlying text rendering in vdp-gl only has partial support for rendering text in variable width fonts. code to handle variable width fonts is only present in the drawText, drawTextWithEllipsis and textExtent canvas API calls

currently all our text rendering is done using drawChar which does not respect character widths - indeed presented with a variable width font, unless the font data has been carefully organised to ensure that all characters are aligned to a standard width, it looks like this call may fail - or at least will draw more than is needed (including a background block for the full width the the font is defined as having)

(it may be the case that characters need to align to standard width)

[stevesims]

canvas->drawChar can be fairly easily updated to support variable width fonts.

alternatively agon-vdp could use canvas->drawGlyph to draw characters rather than drawChar - essentially however this amounts to the same thing, so it is probably better to enhance drawChar