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#141
August 27th 04, 06:15 PM
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"David J Taylor" wrote in message ... Bart van der Wolf wrote: SNIP Careful, the gamma is reversed by the display's non-linear response. True, but what is the incremental contrast sensitivity of the eye - uniform steps in the linear light domain or uniform steps in the log (i.e. gamma corrected) domain? It resembles a kind of logarithmic response, but it's much more complex than that (it also depends on absolute luminance level). But it doesn't matter that much whether the actual scene, or the remapped monitor image stimulates the retina. In both cases the eye will have a similar response if the luminance is the same. SNIP Yes, thanks for pointing those out - I recall seeing at least one of the methods before. I didn't look to see the details of how they had done it, but I suspect they go into a log domain, and reduce the lower spatial frequencies, then return to the linear domain. It is unfortunately much more involved, but it boils down to reducing the gradients of the neigboring pixel differences in a logarithmic luminance version, after which luminance is then linearized (anti-log), gamma adjusted and mapped to the output dynamic range. Large gradients get reduced while maintaining locally enough difference to look natural. In the 2D processing haloing is avoided. It is a very effective technique, though! It makes images as the eye would see them. This http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html is one of my favorite illustrations to demonstrate how inaccurate the human eye is in quantifying absolute luminance. Fortunately the eye/brain adaptation to local area contrast is also the reason that we can exploit it to visualize impossible dynamic ranges on low DR materials. We just need to trick the eye... Bart 
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#142
August 27th 04, 06:15 PM
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"David J Taylor" wrote in message ... Bart van der Wolf wrote: SNIP Careful, the gamma is reversed by the display's non-linear response. True, but what is the incremental contrast sensitivity of the eye - uniform steps in the linear light domain or uniform steps in the log (i.e. gamma corrected) domain? It resembles a kind of logarithmic response, but it's much more complex than that (it also depends on absolute luminance level). But it doesn't matter that much whether the actual scene, or the remapped monitor image stimulates the retina. In both cases the eye will have a similar response if the luminance is the same. SNIP Yes, thanks for pointing those out - I recall seeing at least one of the methods before. I didn't look to see the details of how they had done it, but I suspect they go into a log domain, and reduce the lower spatial frequencies, then return to the linear domain. It is unfortunately much more involved, but it boils down to reducing the gradients of the neigboring pixel differences in a logarithmic luminance version, after which luminance is then linearized (anti-log), gamma adjusted and mapped to the output dynamic range. Large gradients get reduced while maintaining locally enough difference to look natural. In the 2D processing haloing is avoided. It is a very effective technique, though! It makes images as the eye would see them. This http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html is one of my favorite illustrations to demonstrate how inaccurate the human eye is in quantifying absolute luminance. Fortunately the eye/brain adaptation to local area contrast is also the reason that we can exploit it to visualize impossible dynamic ranges on low DR materials. We just need to trick the eye... Bart
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#143
August 27th 04, 07:35 PM
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Bart van der Wolf wrote:
[] This http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html is one of my favorite illustrations to demonstrate how inaccurate the human eye is in quantifying absolute luminance. Fortunately the eye/brain adaptation to local area contrast is also the reason that we can exploit it to visualize impossible dynamic ranges on low DR materials. We just need to trick the eye... Bart What a splendid example! Cheers, David
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#144
August 27th 04, 07:35 PM
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Bart van der Wolf wrote:
[] This http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html is one of my favorite illustrations to demonstrate how inaccurate the human eye is in quantifying absolute luminance. Fortunately the eye/brain adaptation to local area contrast is also the reason that we can exploit it to visualize impossible dynamic ranges on low DR materials. We just need to trick the eye... Bart What a splendid example! Cheers, David
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