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Guide lookup Color look up tables In this chapter ? What is look-up table What is inverse look-up operation ? Preparing color look-up tables ? Building your own custumized object for inverse look-up operaration ? Built-in objects for look-up operations in

Color look up tables In this chapter ? What is look-up table What is inverse look-up operation ? Preparing color look-up tables ? Building your own custumized object for inverse look-up operaration ? Built-in objects for look-up operations in core Java What is a look-up table Why do you need an inverse look-up operation When you want to draw anything on your screen say a uniform gray square at the center of the monitor you do this by creating an object with spatial attributes its location and dimensions etc and with a certain pixel value The position and dimensions of the square are in units of number of pixels which means that their actual size in centimeters will depend on your monitor Similarly the luminance of i e the amount of light emitted by the square will depend not only on the pixel value you assigned but also on your monitor its brightness and color settings and on your video card In a visual psychophysics experiment you naturally want to make sure that the luminance is under your control just as you want to make sure the visual angle the stimulus subtends If you can assume that your monitor ? s output is stable i e the amount of light it emits does not change in time you can measure its output once and then use that measurement as your reference The ?rst step is preparing a look-up table a table which contains the luminance of your display corresponding to each pixel value from to Second upon deciding what luminance to display on your screen you check your look-up table to ?nd the pixel value whose luminance is closest to the luminance you desired to display I refer to this second step as inverse look-up operation Preparing the look-up table Computer monitors have three color channels also called as gun referring to the electron guns in conventional CRT monitors R G and B Therefore you have to prepare a look-up table for each channel In principle one would have to measure and record the luminance for each pixel value at every location on the monitor Not only that one would also have to measure luminance for each channel with varying the other two channels Moreover one would also have to measure luminance at each location in relation to neighboring locations That is a task almost impossible to perform Fortunately there are some sound assumptions you can make to reduce the number of measurements Those assumptions include ? Spatial independence i e output of a pixel at one particular location on the screen doesn ? t depend on the values of other pixels ? Channel constancy i e the relative spectrum of a channel is independent of the pixel value C Color look up tables Figure A plot showing how monitor output typically depends on pixel values ? Channel independence i e one channel ? s output doesn ? t a ?ect the outputs of other two channels ? Spatial homogeneity