[Image-SIG] another out by 0.5 patch
douglas at paradise.net.nz
Fri May 13 03:56:12 CEST 2005
Fredrik Lundh wrote:
>>In my tests, all the resize and stretch filters appear to shift the
>>image down and to the right by about half an output pixel.
> I've attached a (preliminary) patch.
It certainly works for me. Area-averaged and stretched images are
mostly visually indistinguishable -- the exceptions being high contrast
Stretch-resized images are consistently 1/256th darker than the same
images resized by other means. My understanding of this is that when
a region of input pixels is (say) all 255s, except for one pixel of 254,
then the float average is 254.9xx, so the output pixel becomes 254. On
average this should darken by 0.5, but over the two passes, it darkens
by a unit. I've attached a patch.
Here's an example, resizing a 450x325 image to 16x12 (as seen on
http://halo.gen.nz/pil/examples.html), showing mean, RMS, and extreme
differences between pixels.
before the patch, comparing stretched(NEAREST) to gimp:
R: mean: 1.020833 RMS: 1.190238, extrema: (-2.0, 4.0)
G: mean: 0.973958 RMS: 1.120361, extrema: (-1.0, 4.0)
B: mean: 0.911458 RMS: 1.058202, extrema: (-1.0, 3.0)
with the patch:
R: mean: 0.010417 RMS: 0.595119, extrema: (-3.0, 3.0)
G: mean: 0.000000 RMS: 0.595119, extrema: (-2.0, 3.0)
B: mean: -0.08854 RMS: 0.535218, extrema: (-2.0, 2.0)
Contrary to my earlier assertion, I have discovered the area averaged
images differ from the gimp on the bottom row of the image.
comparing area-averaged to gimp:
R: mean: 0.041667 RMS: 0.204124, extrema: (0.0, 1.0)
G: mean: 0.052083 RMS: 0.228218, extrema: (0.0, 1.0)
B: mean: 0.020833 RMS: 0.144338, extrema: (0.0, 1.0)
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