MaskBlt

MaskBlt is a Windows GDI function that performs a masked bit-block transfer from a source device context (DC) to a destination DC. It copies a specified rectangle of pixels from the source to the destination, but only at positions where a provided 1-bit-per-pixel mask bitmap has a set bit. The operation is controlled by a raster operation (ROP) code, which determines how the source and destination pixels are combined on the masked areas.

The function's signature is: MaskBlt(hdcDest, nXDest, nYDest, nWidth, nHeight, hdcSrc, nXSrc, nYSrc, hbmMask, xMask, yMask, dwRop). The mask bitmap is monochrome (1bpp) and dictates which destination pixels are affected. The xMask and yMask parameters specify the offset into the mask, allowing the same mask to be reused for different destination regions.

Common uses of MaskBlt include sprite rendering and image compositing where certain pixels should be preserved (masked off) while others are updated. It is often employed to render images with transparency masks, where the mask defines visible versus transparent areas, combined with a desired raster operation.

Notes and considerations include ensuring the mask dimensions align with the transfer area and understanding that MaskBlt relies on the GDI raster operations. While MaskBlt remains part of the GDI toolkit, modern applications increasingly use alpha blending and Direct2D for more flexible transparency handling. Related functions include BitBlt, TransparentBlt, and StretchBlt.