[GRASS-dev] cause for bad colors in output from png driver

[Glynn Clements]

Michael Barton wrote:

> I finally isolated the cause of the color issue I¹m having with the PNG
> driver.
> 
> It is the GRASS_TRUECOLOR setting.
> 
> If this is set to false, then the PNG looks fine, except that it uses a
> limited color pallet. If it is set to true, the PNG has black where it
> should be white and other issues.
> 
> 
> GRASS_PNG_AUTO_WRITE=TRUE
> export GRASS_PNG_AUTO_WRITE
> GRASS_RENDER_IMMEDIATE=TRUE
> export GRASS_RENDER_IMMEDIATE
> GRASS_TRUECOLOR=TRUE
> export GRASS_TRUECOLOR
> GRASS_PNGFILE=map.png
> export GRASS_PNGFILE
> 
> GRASS 6.3.cvs (spearfish60_test):~ > d.barscale tcolor=green
> PNG: GRASS_TRUECOLOR status: TRUE
> PNG: collecting to file: /Users/cmbarton/map.png,
> GRASS_WIDTH=640, GRASS_HEIGHT=480
> WARNING: Color: can't set color -256
> 
> This gives a black background to the barscale
> 
> GRASS 6.3.cvs (spearfish60_test):~ > d.legend elevation.10m
> PNG: GRASS_TRUECOLOR status: TRUE
> PNG: collecting to file: /Users/cmbarton/map.png,
> GRASS_WIDTH=640, GRASS_HEIGHT=480
> WARNING: Color: can't set color -256
> WARNING: Color: can't set color -16843264
> WARNING: Color: can't set color -33686272
> WARNING: Color: can't set color -50529280
> WARNING: Color: can't set color -50529280
> ...
> A lot more of these errors. This is what is locking up the wxPython overlay
> routine

Duh; this is related to the changes to support mmap()d files.

The framebuffer is always stored as B,G,R,A (the format used by 32-bpp
BMP files). This corresponds to 0xAARRGGBB on a little-endian system
and 0xBBGGRRAA on a big-endian system (e.g. Mac).

Externally (DRV_color, DRV_lookup_color), the alpha value is always
zero (0 = opaque, 255 = transparent), resulting in 0x00RRGGBB on x86
and 0xBBGGRR00 on Mac. But the valid range of colors is always 0 to
0x00FFFFFF inclusive.

Hence the error on Mac, and why I couldn't reproduce it on x86.

I'll change it to always use 0x00RRGGBB externally.

-- 
Glynn Clements <glynn at gclements.plus.com>