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Bitmap size

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Do you mean the size of the image? Are you using the Bitmap class in GDI+? If so, what about using GetWidth() * GetHeight() * bpp ? (You can get the bpp by using GetPixelFormat() and then checking the returned value).

That said, I don't know how the bitmap class stores the data (chances are it uses 4 bpp for a 3bpp image as well, for performace reasons). In addition, images usually have lines longer that xres (or better, xres and a row length don't need to be equal. Most of the times this value is available in the library.

If you want to know the size of the BITMAP object (that is the size of the c++ object), you may use sizeof(BITMAP) but:
-the result will not include pixel data (pixels are refered with a pointer).
-Result also takes into account padding used for alignment (IIRC)

EDIT: if you want to know the size of the image data (first case) another option is to open the file (if it is a bmp) and read the header (BITMAPINFOHEADER) section. From there you should be able to get the pixels data size.

EDIT 2: I've just noticed that there is a BITMAP structure in GDI. Here yopu have a bmWidthBytes value. Multiply it with the bmHeight and you have your pixels size in bytes.

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