Drawbox & drawgrid replace source pixels if input or box color has alpha

[Gyan]

If the video input has an alpha component or the user specifies transparency in the box/grid-color, the drawbox and drawgrid filters replace the source pixels when painting the box or grid. This is almost always undesired. The alpha component of the user color is meant for blending the box/grid on top of the input. The input alpha should be left intact. Essentially, the filters draw a "hole" if alpha is involved. See attached result-drawbox-src-with-no-alpha-existing.jpg

Inputs and outputs for

ffmpeg -i view.jpg -i window.png -lavfi "[1]drawbox=200:0:400:56:c=black@0.5:t=fill[win];[0][win]overlay" result-drawbox-src-with-alpha-existing.jpg

are attached, along with expected result.

[Gyan]

Patch at ​https://patchwork.ffmpeg.org/patch/6701/ introduces a new boolean option 'replace'. If set to 1, yes or true, current behaviour is retained. With the default of 0, no or false, the behaviour is as shown in the 'expected' image.

[llogan]

Fixed by your patch in 1c76134fe37ac20695627e3f5ce1f2bbf1245fcc. Thanks.