CODEWITHSUNDEEP
winapibitblt
pete
pete

Reputation: 2046

Very unexpected behavior of C++ win32 BitBlt

I noticed when I try to run BitBlt, the resulting data buffer is unexpected in two ways:

For the first issue, I noticed it when testing with my command prompt; if my command prompt was in the upper left portion of the screen, it would only say it was black when my cursor was in the lower left portion. I had to fix the inversion of the y axis by changing int offset = (y * monitor_width + x) * 4; to int offset = ((monitor_height - 1 - y) * monitor_width + x) * 4; this fixed the pixel location issue because it was showing black where I expected black.

However, the colors were still strong. I tested by trying to get the color of known pixels. I noticed every blue pixel had a very high R value and every red pixel had a very high blue value. That's when I compared with an existing tool I had and found out that the red and blue values seem to be switched in every pixel. At first I thought it was backwards or a byte alignment issue, but I also verified in a clustering of pixels that aren't uniform to make sure it's picking the right position of pixel, and it did perfectly well, just with the colors switched.

Full simplified code below (originally my tool was getting my cursor position and printing the pixel color via hotkey press; this is a simplified version that gets one specific point).

BYTE* my_pixel_data;

HDC hScreenDC = GetDC(GetDesktopWindow());
int BitsPerPixel = GetDeviceCaps(hScreenDC, BITSPIXEL);
HDC hMemoryDC = CreateCompatibleDC(hScreenDC);
int monitor_width = GetSystemMetrics(SM_CXSCREEN);
int monitor_height = GetSystemMetrics(SM_CYSCREEN);
std::cout << std::format("monitor width height: {}, {}\n", monitor_width, monitor_height);
BITMAPINFO info;
info.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
info.bmiHeader.biWidth = monitor_width; // client_width;
info.bmiHeader.biHeight = monitor_height; // client_height;
info.bmiHeader.biPlanes = 1;
info.bmiHeader.biBitCount = BitsPerPixel;
info.bmiHeader.biCompression = BI_RGB;

HBITMAP hbitmap = CreateDIBSection(hMemoryDC, &info, DIB_RGB_COLORS, (void**)&my_pixel_data, 0, 0);
SelectObject(hMemoryDC, hbitmap);

BitBlt(hMemoryDC, 0, 0, monitor_width, monitor_height, hScreenDC, 0, 0, SRCCOPY);

int x = 12, y = 12;
int offset = ((monitor_height - 1 - y) * monitor_width + x) * 4;
std::cout << std::format("debug: ({}, {}): ({}, {}, {})\n", x, y, (int)my_pixel_data[offset], (int)my_pixel_data[offset + 1], (int)my_pixel_data[offset + 2], (int)my_pixel_data[offset + 3]);

system("pause");

The output of this will be debug: (12, 12): (199, 76, 133) even though another program has verified the colors are actually (133, 76, 199).

I can easily fix this in my code by flipping the y axis and switching each R and B value and the program will work perfectly well. However, I am just baffled by how this happened and whether there's a more elegant fix.

Upvotes: 0

Views: 783

Answers (1)

Zaren
Zaren

Reputation: 1

I can answer the RGB (and it looks like Hans answered the inverted Y axis in a comment). Remember that RGB is stored 0xAARRGGBB, so in that 32 bit value BB is byte 0, GG is byte 1, and RR is byte 2 (alpha is byte 3 if you use it), so when you index in at +0, +1 and +2 you're actually getting the values correctly. When we say RGB we're saying the colors in opposite order of how they're stored in memory.

Upvotes: 0

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