On some screens (typically low quality laptop screens), using Bayer
ordered dithering has been observed to cause color changes depending on
*where the gradient is rendered on the screen*, causing visible
flickering when moving an image on the screen.
To alleviate the issue, this patch adds support for ordered dithering
using a 64x64 matrix tuned from blue noise. In addition to being devoid
of the positional dependency on screen, the blue noise matrix also
generates more pleasing and less discernable patterns. As such, it is
now the method used for PIXMAN_DITHER_GOOD and PIXMAN_DITHER_BEST
dithering methods.
The 64x64 blue noise matrix has been generated using the provided
`pixman/dither/make-blue-noise.c` script, which uses the
void-and-cluster method.
Changes since v1 (thanks Bill):
- Use uint16_t for the blue noise matrix for lower memory usage
- Use bitwise computation for array index
This adds support for testing dithered destinations in tolerance-test.
When dithering is enabled, the pixel checker allows for an additional
quantization error.
Compiling with the Intel Compiler reveals a problem:
tolerance-test.c(350): error: index variable "i" of for statement following an OpenMP for pragma must be private
# pragma omp parallel for default(none) shared(i) private (result)
^
In addition to this, the 'result' variable also should not be private
(otherwise its value does not survive after the end of the loop). It
needs to be either shared or use the reduction clause to describe how
the results from multiple threads are combined together. Reduction
seems to be more appropriate here.
This new test program is similar to test/composite in that it relies
on the pixel_checker_t API to do tolerance based verification. But
unlike the composite test, which verifies combinations of a fixed set
of pixels, this one generates random images and verifies that those
composite correctly.
Also unlike composite, tolerance-test supports all the separable blend
mode operators in addition to the original Render operators.
When tests fail, a C struct is printed that can be pasted into
pixel-test for regression purposes.
There is an option "--forever" which causes the random seed to be set
to the current time, and then the test runs until interrupted. This is
useful for overnight runs.
This test currently fails badly due to various bugs in the blend mode
operators. Later commits will fix those.
