Scanning Data: DS9 needs to determine the min and max data values to correctly display your image. Allocate is very slow, limit is amount of physical memory + swap partition. Memory Map ( mmap) is very fast, limit is memory address space (see above). $ xpaset -p ds9 fits foo.fits # allocates memory $ xpaset -p ds9 file foo.fits # uses mmap $ cat foo.fits | ds9 - # allocates memory Memory Management: There are a number of memory management techniques supported in DS9 that will greatly affect the ability and speed of loading large files: $ ds9 foo.fits # uses mmap Two parameters, r (radius) and k (kernel size ), are defined as the following: Kernel width/height = 2k+1 with r. The user may select one of three types of smoothing kernels. The histogram equalization scale function distributes colors based on the frequency of each data value. The sinh scale function is defined as the following: The asinh scale function is defined as the following: The square scale function is defined as the following: The sqrt scale function is defined as the following: The user may specify an exponent a to change the distribution of colors within the colorbar. The pow function is defined as the following:Īs x goes from 0 to 1. A value of 10000 closely matches the log function of SAOImage as defined as the following: Typically, optical images respond well at 1000, IR images as low as 100, and high energy bin tables up to 10000. The log function is defined as the following:Īs x goes from 0 to 1. Based on your data clip values, and the value of the pixel you have, index into the result of step 4, and you have an index into lookup table (for pseudocolor) and an RGB pair (for truecolor and postscript). This involves taking the result of step 2, and creating yet another array, this time of size 16384, redistributing the colors, based on the scale algorithm selected (see Scales). The min/max data values may be used or an algorithm may be used to determine the clip data values. Calculate the data clip values (low/high data values). The length of the new array will between 200 (for pseudocolor) and 4096 (for truecolor). This step takes the result of step 1 and creates a new array with the contrast/bias applied. ) or the user may load his own color scale. DS9 contains a number of predefined color scales (Gray, A, B, I8. The number of RGB triplets can vary from just a few to over 200. A color scale is defined as a number of colors (RGB triplets). you need a color scale, a contrast/bias pair for the colorscale, clip values for the data, a scale distribution, and finally, the value of the pixel in question. Here is a short description on how DS9 decides to paint a pixel a color on the the screen, give an data value. Table of Contents How DS9 Renders an Image
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