Difference between revisions of "Raw preprocessing"
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* apply a clip frame in overexposed highlights: | * apply a clip frame in overexposed highlights: | ||
im(im > clip_thr) -= clip_frame(im > clip_thr) % clip_thr is about 2500 for gain x1; smooth transition might be also needed | im(im > clip_thr) -= clip_frame(im > clip_thr) % clip_thr is about 2500 for gain x1; smooth transition might be also needed | ||
* apply a look-up table (either per-channel or global): | |||
im = lut(im) | |||
That's it. | That's it. | ||
Doing the math on 16 bits is preferred, and when getting back to 12 bits, add one bit of random noise (better results). See pack12/unpack12 on raw2dng for details on the implementation, and http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p3.html for the rationale behind it. | Doing the math on 16 bits is preferred, and when getting back to 12 bits, add one bit of random noise (better results). See pack12/unpack12 on raw2dng for details on the implementation, and http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p3.html for the rationale behind it. |
Latest revision as of 17:55, 18 January 2016
1 Row noise correction using black reference columns
Using octave/matlab-like pseudocode.
1.1 For previous frame, compute:
For each row:
med_left(y) = median(im(y,1:8)) % median for first 8 columns med_right(y) = median(im(y,end-7:end)) % median of last 8 columns
For the entire image:
offset_odd_left = median(med_left(1:2:end)) % median of medians from odd left rows offset_odd_right = median(med_right(1:2:end)) % median of medians from odd right rows offset_even_left = median(med_left(2:2:end)) % median of medians from even left rows offset_even_right = median(med_right(2:2:end)) % median of medians from even right rows
1.2 For current frame:
1. Subtract a dark frame (constant image)
im -= dark_frame
2. Fix black level offsets:
- these are expected to change very slowly over time, not from frame to frame, so we are reusing them from previous frame to avoid multiple passes
- subtract an offset for odd rows and another offset for even rows
- the offset varies linearly from left to right
- we will attempt to keep a constant black level: let's say target_black_level = 128
- subtract these values from each row:
offset_odd(x) = offset_odd_left + (offset_odd_right - offset_odd_left) * x / width - target_black_level; offset_even(x) = offset_even_left + (offset_even_right - offset_even_left) * x / width - target_black_level; im(1:2:end,x) -= offset_odd(x) im(2:2:end,x) -= offset_even(x)
3. Fix dynamic row noise
- this changes with every frame, and is not correlated from frame to frame
- we have a noisy estimation of this row noise in the black reference columns
- average black columns (16 values for each row):
mb(y) = mean(black_columns)(y) = mean(im(y, [1:8 end-7:end])
- from each row, we will subtract a scalar value: k * mb(y), where k is 0.6 for gain x1:
im(y,:) -= k * mb(y)
- proof that 0.6 is optimal: https://github.com/apertus-open-source-cinema/misc-tools-utilities/commit/48de47b2a544dc32bbd5a8fd7701bb44a31ea850#diff-624053a553f49c0036b4d31282e58b2fR301
Optional steps (to be tested):
- apply a gain frame:
im = (im - target_black_level) .* gain_frame + target_black_level
- apply a clip frame in overexposed highlights:
im(im > clip_thr) -= clip_frame(im > clip_thr) % clip_thr is about 2500 for gain x1; smooth transition might be also needed
- apply a look-up table (either per-channel or global):
im = lut(im)
That's it.
Doing the math on 16 bits is preferred, and when getting back to 12 bits, add one bit of random noise (better results). See pack12/unpack12 on raw2dng for details on the implementation, and http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p3.html for the rationale behind it.