SetDIBitsToDevice emulation

Question

I'm gho author of the DxWnd tool that tries to provide support to run old and legacy programs on modern windows environment.

A very strange game is G-Nome that, despite being able to run natively on Win7-10 systems, brings a serious problem for graphic manipulation.

The game links to ddraw, but builds every frame as a DIB_PAL_COLORS DIB and brings the DIB to screen using SetDIBitsToDevice user32 calls. The operation is supported by OBJ_DC type DC on video also on 32 bit color depth video modes, but produces bad colors when applied to MEM_DC type DC. I found this while trying to scale the DIB using a typical schema like this (variable declarations and error checking stripped for brevity):

hTempDc=CreateCompatibleDC(hdc);
hbmPic=CreateCompatibleBitmap(hdc, OrigWidth, OrigHeight);
SelectObject(hTempDc, hbmPic);
SetDIBitsToDevice(hTempDc, 0, 0, OrigWidth, OrigHeight, XSrc, YSrc, uStartScan, cScanLines, lpvBits, lpbmi, fuColorUse);
StretchBlt(hdc, XDest, YDest, dwWidth, dwHeight, hTempDc, 0, 0, OrigWidth, OrigHeight, SRCCOPY);

According to MSDN, when you output a DIB_PAL_COLORS DIB the pixels are mapped to the "current" realized palette, but of course on a 32 bit surface and on its memory DC clone there is no palette at all, so it's no wonder that the pixel colors look all wrong.

An alternative solution could be to emulate the SetDIBitsToDevice behaviour building a 32 bit memory DC to be then scaled at will with StretchBlt, but again it is not documented how I could convert a 8bpp DIB_PAL_COLORS DIB to either a 32bpp DIB_RGB_DIB or a 32bpp DC.

I already tried in several ways, but in all cases it seems that I can't identify the correct palette to be used to convert pixel indices. Could someone help me to interpret the 8bpp DIB_PAL_COLORS DIB format?

Answer 1

If I understand the question correctly, you have a source image in the form of a DIB with a color table and you want to get that image onto a non-palette DC using SetDIBitsToDevice.

I haven't worked with palettes in a very long time, but, if I recall correctly, you cannot use DIB_PAL_COLORS unless the destination DC is a palette device. It seems unlikely that your memory DC is a palette device, but you can check:

GetDeviceCaps(hTempDc, RASTERCAPS) & RC_PALETTE

Assuming it's not a palette device, I think you have to use DIB_RGB_COLORS for fuColorUse. SetDIBitsToDevice will use each pixel's index to look up the corresponding RGBQUAD value in the color table. (The color table comes after the BITMAPINFOHEADER in the BITMAPINFO pointed to by lpbmi.) The RGBQUAD values are applied pixel-by-pixel to the destination device.

If you still have problems, I'd look closely at the BITMAPINFO and the color table you're providing to make sure they correctly represent the format of the pixel data and the corresponding color table.

Answer 2

Oh, I think I got the culprit: another DxWnd hooker was faking the RC_PALETTE capability in GetDeviceCaps(). Removing that fake capability the game no longer tries to use DIB_PAL_COLORS format and builds more manageable DIB_RGB_COLORS DIBs. Problem fixed, sorry for the false track and thank you for helping.