My question is can these filters be re-written in terms of Core
Image? The reason I ask is that the simple examples I've seen so
far are of the 'Apply this algorithm to each pixel in the image'
variety. I want something a bit different: an ordered dither for
example. To do an ordered dither I need to divide the image into
squares (4x4 say); count how many pixels in a square are above a
threshold; set pixels within the square depending on how many were
above the threshold.
In general, you cannot pass any information from one pixel to
another. So the usual algorithm for, say, a Floyd-Stienberg dither
can't be done in Core Image.
There may be some way to work around the limitation, though. For
example, for each pixel in the output, sample the surrounding input
pixels that would normally be 0 in the Floyd-Stienberg kernel,
dither each one to figure out the error propogated from each one,
add those values together, add them to the current pixel, and round
it off as appropriate. It means you'll be sampling a bunch of times
more than you would with the traditional algorithm, so it might not
be very fast. And I can't verify that my method will give identical
results. :-) But it's worth a try.
<sound of sucking teeth> Not sure about that - you'll be losing some
of the errors! (Normally losing errors sounds like a good idea, but
not here).
Anyway, straight ordered dither, unlike Floyd Steinberg doesn't
require passing information from pixel to pixel and can be done by
only looking at a single pixel at a time so you should be able to do
it in Core Image. The difficulty may be in passing in the dither
matrix. I've not looked at Core Image for some months now, so I may
be making this up, but you could pass a 4x4 dither matrix in easily
enough using vec4s, say. For larger matrices, you may have to do
something like make the matrix into an image and use a sampler.
Jerry
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