32bpp and OpenTTD
Last updated:    2007-06-13
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Table of contents
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1.0) About
2.0) The format


1.0) About
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OpenTTD has 32bpp support. This means: OpenTTD still is 8bpp, but it has the
posibility to override the graphics with 32bpp. This means that it isn't a
replacement of grf or newgrf, but simply an addition. If you want to use 32bpp
graphics of a newgrf, you do need the newgrf itself too (with 8bpp graphics).


2.0) The format
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32bpp images are stored in PNG. They should go in:
  data/sprites/<grfname>/<SpriteID>.png

For example, a grfname would be 'openttd' (without .grf, lowercase), and the
SpriteID 3, to override the 3rd sprite in openttd.grf with a 32bpp version.

The format of this PNG can be almost anything, but we advise to use RGBA
format. Alpha-channel is fully supported.

As the core of OpenTTD is 8bpp, and because you of course want company colours
in your images, you will need to add a mask for every sprite that needs colour
remapping. The name is simular as above, only you have to put a 'm' behind the
SpriteID. This image should be a 8bpp palette image, where the palette is the
OpenTTD palette. Upon load of the PNG, the mask is loaded too, and overrides
the RGB (not the Alpha) of the original PNG image, and replacing it with a
8bpp color remapped and converted to 32bpp.

Another thing that OpenTTD needs in your png, is 2 tEXt chunks: x_offs and
y_offs. This to define the x- and y-offset, of course. Use the tool we supply
to add this information. Sadly enough most graphical editors trashes those
chunks upon save, so you have to readd it every time you save your image.

Your images should be the same as the grf, in size.