To: Steven Dakin <s.dakin@...>
From: Denis Pelli <denis.pelli@...>
Hi DenisFurther to my question of the other day (what's the rule for combining RGB gamma functions to predict luminance with the attenuator?) I have one more general question. The philosophy when using the attenuator seems to be "vary as few DACs as possible" to minimize accumulating error between their outputs. However if one varies (say) the G and B DACs while fixing B at (say) 128, isn't the precision of the resulting luminance still dependent on the precision of the gun carrying the largest portion of the luminance (i.e. B)? Given that the thing works, am I to take it that DACs tend to be inaccurate but quite precise?This isn't idle curiosity. I'm trying to assess the feasibility of making a "thousands of grey levels" display using 24-bit SCREEN mode. The programming is straightforward: construct a thousands-of-entries LUT, and a thousands-of-luminances image, then use the image to offset the CLUT to generate an RGB triplet that would give the desired luminances (three or four lines in MatLab once you've made the CLUT).The advantages of such a working system are obvious: e.g. very high-contrast and very low-contrast stimuli could be presented simultaneously, natural images could be rendered in 12-bit, etc.Do you know if anyone's tried this?Thanks in advance (as always) for any thoughts you have on thisAll the bestSteven--Steven DakinInstitute of OphthalmologyUniversity College London11-43 Bath StreetLondon EC1V 9ELT: +44 20 7608 6988F: +44 20 7608 6850W: http://www.homepages.ucl.ac.uk/~smgxscddear stevenhttp://vision.nyu.edu/Tips/Attenuator.htmlmost of what you say is didactic and correct.isn't the precision of the resulting luminance still dependent on the precision of the gun carrying the largest portion of the luminance (i.e. B)?yes. but typically we care much more about accuracy of contrast than of luminance. A fixed luminance error across the screen and throughout the whole presentation of even as much as a few percent would usually be considered negligible. What matters is that the step sizes of the increments and decrements that form your (say) 0.3% contrast grating be accurate.if you want thousands of colors in your palette i suggest buying a card with a 10-bit clut and dac. ATI Radeon 7000. it costs only $130 and will be wholly compatible with the Pelli-Zhang attenuator.http://psychtoolbox.org/mac.html#driversYou can select any contrast range you like and design optimal settings for that range. However, the achievable contrast accuracy will be better if the range is smaller. It's not clear to me what you expect to achieve from your proposed scheme above, but what you propose does not escape the tradeoff between range and accuracy. Number of settings is not the bottleneck; the bottleneck is whether you change the coarsest DAC or not. If it changes, then parts of the display will have errors that could differ by up to a whole LSB of that DAC.you COULD overcome that by calibrating the coarse DAC, measuring its "errors" once and for all, and taking them into account. Then there would be no error.bestdenis