Peter Yang wrote:
> Here is situation..
>
> 1. Draw rectangle A with Solid Color RGBA(0,0,255,128)
> 2. Draw rectangle B with solid color RGBA(0,255,0,128) overlapped
> with A
> 3. Draw rectangle C -RGBA(0,255,0,128) in different DIB buffer
> and blend over rectangle A
>
> all pixel format was agg::pixfmt_bgra32
>
> overllapped area between 1 and 2, 1 and 3 produce different color :(..
This is because your buffers are premultiplied, i.e. for each pixel the RGB values are scaled by the alpha value.
pixfmt_bgra32 blends "plain" (non-premultiplied) source with premultiplied destination.
pixfmt_bgra32_pre blends premultiplied source with premultiplied destination.
pixfmt_bgra32_plain blends plain source with plain destination.
Therefore, to blend two premultiplied buffers you need to use pixfmt_bgra32_pre.
- Jim
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