Windows bitmaps, icons and cursors

Implemented in colwin/formats/dib.py. All of these live in COLONIZE.EXE and SETUP.EXE — they are the Windows interface chrome, not game art.

RT_BITMAP — round-trips

Six 32×32 4-bpp DIBs with a BITMAPINFOHEADER, ids 301–306. Ordinary indexed images carrying their own palette, so they extract and re-import the same way a canvas does.

Scanlines are bottom-up and padded to a 4-byte boundary. The palette is stored BGR(A), not RGB.

One detail worth naming, because getting it wrong is invisible until you compare bytes: biClrImportant is 16 in bitmap 306 and zero in the other five. An encoder that writes a constant there produces a file that decodes identically and is not the same file. colwin preserves biXPelsPerMeter, biYPelsPerMeter, biClrUsed and biClrImportant from the manifest, which is what takes the six bitmaps from five byte-exact to six.

RT_ICON and RT_CURSOR — read only

An icon is a colour image and a 1-bpp AND mask stacked in one DIB of doubled height. The header declares one bit depth and twice the real height, but only the top half is at that depth:

Reading the whole thing at the declared depth is what makes an icon look truncated — it asks for twice the data that is there.

colwin decodes both for viewing, writing RGBA PNGs into reference/ with the AND mask applied as alpha, and carries the original bytes through on a rebuild. Putting an edited alpha channel back into a 1-bpp mask plus an indexed image is a lossy choice, and not one to make on the artist’s behalf without being asked.

RT_MENU, RT_DIALOG, RT_GROUP_ICON, RT_GROUP_CURSOR

Carried through untouched.