Bug 310211

When doing subtle curve corrections to 16-bit images, the histogram often shows damage caused by the curves correction. The resulting histogram looks 'hairy' (see attachment), which suggests that the curve correction calculation is done at 16 bits per channel. This lack of precision causes many different input values to yield the same output value, which explains the spikes on the histogram.

I have not seen any visible effects of the damage caused by the curves plugin, but it might be easy to fix:

1. Take the 16-bit input value, store in 32-bit variable.
2. Bitshift the 32-bit value to the left by 16 places
3. Add a random 16-bit value
4. Apply curves correction function, yields 32-bit output value
5. Bitshift the 32-bit output value to the right by 16 places
6. Store 32-bit output value in 16-bit variable

The same applies to curve corrections to 8-bit images, which requires 16 bits of precision.


Reproducible: Always