StretchBlt only works when nHeightDest is negative

Question

I'm trying to use StretchBlt in order to copy pixels from a memory hdc to the window hdc.
The memory hdc gets the image from an invisible window which renders a stream using openGL.

Here's my code:

BITMAPINFOHEADER createBitmapHeader(int width, int height) {
    BITMAPINFOHEADER header;
    header.biSize = sizeof(BITMAPINFOHEADER);
    header.biWidth = width;
    header.biHeight = height;
    header.biPlanes = 1;
    header.biBitCount = 32;
    header.biCompression = BI_RGB;
    header.biSizeImage = 0;
    header.biXPelsPerMeter = 0;
    header.biYPelsPerMeter = 0;
    header.biClrUsed = 0;
    header.biClrImportant = 0;

    return header;
}

...

HDC memoryHdc = CreateCompatibleDC(windowHdc);
BITMAPINFO bitmapInfo;
bitmapInfo.bmiHeader = createBitmapHeader(targetDimensions.width, targetDimensions.height);
HBITMAP bitmap = CreateDIBitmap(windowHdc, &bitmapInfo.bmiHeader, CBM_INIT, offscreenBuffer, &bitmapInfo, DIB_RGB_COLORS);
SelectObject(memoryHdc, bitmap);
DeleteObject(bitmap);

SetStretchBltMode(windowHdc, COLORONCOLOR);
StretchBlt(windowHdc,
    targetDimensions.x, targetDimensions.y,
    targetDimensions.width, -targetDimensions.height,
    memoryHdc,
    sourceDimensions.x, sourceDimensions.y,
    sourceDimensions.width, sourceDimensions.height,
    SRCCOPY);

DeleteDC(memoryHdc);

Where windowHdc is the hdc of the window to which I want the StretchBlt to copy the pixels to, and offscreenBuffer is a void* to the pixels copied from the offscreen window in which the openGL is rendering.

This code works great, except that the image is upside down and I want it vertically flipped.
I know that this happens because:

If nHeightSrc and nHeightDest have different signs, the function creates a mirror image of the bitmap along the y-axis

But when I remove the minus sign and both target and source heights are the same then I see no image in the window.
Just to check, I tried to put the minus on the sourceDimensions.height but that also results in no image, and the same if I try to negate the widths (both target and source).

Any idea why?
Thanks.

If I change my `createBitmapHeader` function so that `header.biHeight = -height` (and keep the `targetDimensions.height`) to negative then the image is indeed the way I want it (not upside down), but it renders below where it used to before (probably targetDimensions.height difference). Since you suggested this, do you have any idea why does this happen? Thanks — Nitzan Tomer, Aug 03 '15 at 09:44
[BITMAPINFOHEADER structure](https://msdn.microsoft.com/en-us/library/dd183376.aspx): *"If **biHeight** is positive, the bitmap is a bottom-up DIB and its origin is the lower-left corner. If **biHeight** is negative, the bitmap is a top-down DIB and its origin is the upper-left corner."* — IInspectable, Aug 03 '15 at 10:58
@IInspectable Thanks for the info, that explains the change when changing the height in the `BITMAPINFOHEADER` struct, but why isn't the `StretchBlt` working (or at least I can't see the output) when both the source and target heights are positive? — Nitzan Tomer, Aug 03 '15 at 11:47

StretchBlt only works when nHeightDest is negative

0 Answers0