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20161101 - Linear Dithering Before Transfer Function
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Source Material
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Fine Art of Film Grain
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Grain and Fine Details
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Advanced Techniques and Optimization of VDR Color Pipelines - Link to GDC presentation on this page
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Extra Notes
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+The point of
Fine Art of Film Grain
+was to show how to take an artist generated grain texture,
+specifically something which isn't a perfect repeating blue noise texture,
+and make it energy conserving so it can serve double usage as video grain and dithering.
+
+Bart Wronski's
Dithering Part Two - Golden Ratio Sequence, Blue Noise and Highpass and Remap
+qualifies the frequency domain output of repeated highpass with remap used to reshape a poor quality noise.
+The technique I use to generate an artistic grain texture is a little different,
+I would do many repeated highpass passes,
+and only at the end do one remap step.
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+
Linear Dither Before Non-Linear Quantization
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+
+My VDR GDC presentation talked about this,
+but the slides don't go into enough detail on why.
+This as an alternative method of quantization to the traditional quantization in the non-linear output space.
+Both have various cost/quality trade-offs.
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+
+In order for a dither to be energy conserving,
+meaning not introduce any bias into the signal temporally,
+on a still image,
+the average of the output values over time should be the non-dithered input.
+Best way to test this is to try to dither to 2-bits/channel on a color photograph with a temporal dither.
+If the photo's contrast or saturation changes, then a bias had been introduced in the signal.
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+Near black, bias can also be a large problem.
+To avoid any bias, over time,
+any amount of positive dithering must be matched with a linearly equal amount of negative dithering temporally.
+However the display and signal cannot reproduce negative luma. So dithering must be adjusted as image luma approaches zero (talked about around slide 95-96 in my GDC presentation).
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+In a 2-bit/channel dithering ground truth test, a similar adjustment must be done to the whites.
+However in practice I don't do the correction for whites with typical 8-bit output,
+because I don't feel it is perceptually important. It is perceptually important in the darks,
+and note be very careful here, because some displays like WRGB OLED TVs clamp a large range of darks to zero by default.
+Evaluation of darks need to be done in a theater black ambient level room with a very low APL scene on a calibrated display which is actually capable of correct black output close to zero.
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+Hardware ROP rounds to nearest in the non-linear output space.
+Linearly speaking, this rounding is biased where the bias changes based on the slope of the transfer function for the non-linear output space.
+Likewise if a dither is added after the linear to non-linear transform,
+then equal positive and negative dither contribution to that non-linear signal before quantization will also have bias
+(which changes based on the slope of the transfer function).
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+While it is possible to correct for bias with dithering in the non-linear output space,
+I find it excessively expensive to do so in real-time.
+The solution I use instead is to introduce dither linearly with enough dither so that there are no bands of unchanging values,
+and so that the final non-linear quantized value is temporally switching between at least 2-3 values.
+Given the linear dithered value,
+it is possible to do linear nearest quantization in the shader instead of letting the hardware do non-linear nearest in say a 10:10:10:2 non-linear output.
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+Lastly, when adding grain/dither linearly with output PQ2048 transfer functions,
+an energy conserving asymmetrical grain distribution must be used (covered around slide 99 in my GDC presentation).
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