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Up-to-date documentation:

Deprecated: see discussion

FixLuminance(clip clip, int intercept, int slope)

int  intercept = (required)
Bottom line at which to start correction. intercept should be equal to or smaller than the height of the image.
int  slope = (required)
Slope of correction function. The top of the picture is made darker by 1 (on a 0-255 scale) every slope lines.
Maximum darkening (at top line) = intercept / slope

The purpose of this filter is to progressively darken the top of the image; for example, to compensate for certain VCRs which make the top of each frame brighter than the bottom. In practice it only works for NTSC content and it needs its fields separated.

When applying it to an image with height > 255 pixels, you will see a repetitive gradient. So it doesn't work for field-separated PAL content.

This filter works requires YUY2 input.


  • Progressively darken top lines.
Crop(0, 0, 0, 256)
FixLuminance(100, 4)

Line 99 is normal; line 0, the topmost line, is darker by 25 (intercept/slope = 100/4 = 25)

  • Make a ramp test image.
BlankClip(height=256, pixel_type="YUY2", color_yuv=$ff8080)
FixLuminance(255, 1)


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