Direct Access to CreateDIBitmap Bits - winapi

Question

[The final fix, which works unconditionally: use SetDIBitsToDevice, not BitBlt, to copy out the post-text-draw image data. With this change, all occurrences of the problem are gone.]
I fixed the problem I'm having, but for the life of me I can't figure out why it occurred.
Create a bitmap with CreateDIBitmap. Get a pointer to the bitmap bits.
Select the bitmap into a memory DC.
Background fill the bitmap by directly writing the bitmap memory.
TextOut.
No text displays.
What fixed the problem: change item 3. from direct fill to a call to FillRect. All is well, it works perfectly.
This is under Windows 10 but from what little I could find on the web, it spans all versions of Windows. NO operations work on the bitmap - even calling FillRect - after the manual write. No savvy, Kimosabe. Elsewhere in the app, I even build gradient fills by directly writing to that bitmap memory and there is no problem. But once TextOut is called after the manual fill, the bitmap is locked (effectively) and no further functions work on it - nor do any return an error.
I'm using a font with a 90 degree escapement. Have not tried it with a "normal" font, 0 degree escapement. DrawTextEx with DT_CALCRECT specifically states it only works on 0 degree escapement fonts so I had to use TextOut for this reason.
Very bizarre.
No, there were no stupid mistakes like using the same text color as the background color. I've spent too long on this for that. One option people have available is that the endless energy that would normally be spent destroying the question and/or the person who asked it could instead be used to write a few lines of code and try it for yourself.

Answer 1

Here's a function to make a bitmap. Don't pass a plain colour, pass a gradient fill, say going from white to pinkish.
Does it display correctly? If so, does the TextOut call on top of that work?
static HBITMAP MakeBitmap(unsigned char *rgba, int width, int height, VOID **buff)
{
VOID *pvBits; // pointer to DIB section
HBITMAP answer;
BITMAPINFO bmi;
HDC hdc;
int x, y;
int red, green, blue, alpha;
// setup bitmap info
bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bmi.bmiHeader.biWidth = width;
bmi.bmiHeader.biHeight = height;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biBitCount = 32; // four 8-bit components
bmi.bmiHeader.biCompression = BI_RGB;
bmi.bmiHeader.biSizeImage = width * height * 4;
hdc = CreateCompatibleDC(GetDC(0));
answer = CreateDIBSection(hdc, &bmi, DIB_RGB_COLORS, &pvBits, NULL, 0x0);
for (y = 0; y < height; y++)
{
for (x = 0; x < width; x++)
{
red = rgba[(y*width + x) * 4];
green = rgba[(y*width + x) * 4 + 1];
blue = rgba[(y*width + x) * 4 + 2];
alpha = rgba[(y*width + x) * 4 + 3];
red = (red * alpha) >> 8;
green = (green * alpha) >> 8;
blue = (blue * alpha) >> 8;
((UINT32 *)pvBits)[(height - y - 1) * width + x] = (alpha << 24) | (red << 16) | (green << 8) | blue;
}
}
DeleteDC(hdc);
*buff = pvBits;
return answer;
}