***************************** * * * Color Table Adjustments * * * ***************************** ************* * * * General * * * ************* While there are 256 colors available at any time, only about 200 are available for mapping to the pixel values of the image. The remainder have already been reserved by the program for the menus, outlines, labels, etc. The 200 colors are "computed" from a look-up-table (lut). Several luts are available and may be selected via the "Options" submenu with the "Colormap" button or via the Color Palette Menu button at the bottom of the Color Table Adjustments menu. Different luts represent different arrangements of colors and color ranges. For example, the default lut is a gray scale. Other examples would be a blue scale of intensities or a palette of 20 colors each representing a 5% range of image pixel values or a black body spectrum ranging from high intensity white-yellows through low intensity orange-blacks. Different luts may be more meaningful for different people with different images. Selection of a new color lut resets the modes, guns, and sliders in the Color Table Adjustments menu to their default positions. *********** * * * Modes * * * *********** Normal (NML): the color table is as specified in the default or selected color look-up-table. Invert (INV): the intensities (range 0.0 to 1.0) are subtracted from 1.0, giving the appearance of different colors. For example, white becomes black, red becomes cyan, blue becomes yellow, green becomes magenta, etc. Note that an intensity change means a change for all three R-G-B guns. Reverse (REV): the arrangement of the color table is reversed. That is, if there are 200 positions, the colors at positions 0 and 199 are swapped, the colors at positions 1 and 198 are swapped, etc. The reverse is seen most easily in the colorbar where top and bottom are flipped. Note there are 2 columns in the Modes section of the menu. On the left, the "Pick" means to select one of the 3 items in the list. The most recent selection is highlighted. The default selection is Normal (NML). On the right, the "Status" means the current state of the table, which may or may not correspond to the most recent selection. Status labels "N"(Normal) "I"(Inverted) "R"(Reversed) are highlighted to reflect the current status. For example, if Invert (INV) is selected twice in a row, the Invert (INV) list item will be highlighted, but the status would have changed from N to I and back to N. For another example, the table may be in a state to be inverted and reversed at the same time. Here is a table of mode status changes: Current Result Status Pick Status ------- -------- ------ N - - Normal N - - - I - Normal N - - - - R Normal N - - - I R Normal N - - N - - Invert - I - - I - Invert N - - - - R Invert - I R - I R Invert - - R N - - Reverse - - R - I - Reverse - I R - - R Reverse N - - - I R Reverse - I - Disallowed Status ----------------- N I - N - R N I R - - - ********** * * * Guns * * * ********** When any slider or sliders are moved, the color table intensity values are changed. As a default, all three guns (red, green, blue) are affected the same way. However, selection of any of the individual red, green, or blue guns in the "Pick Guns" menu means that only the selected gun will be affected by subsequent slider movements. The choices are all three guns OR any one of the three. The current version of this program does not allow for any two. ************* * * * Sliders * * * ************* When any slider or sliders are moved, the color table intensity values are changed according to the slider position. Values for a slider's position run from 0.0 (left) to 0.5 (middle) to 1.0 (right). Definition Initial Minimum Maximum ----------------------------- ------- ------- ------- gamma = (2*position) * (2*position) 1.0 0.0 4.0 contrast = (2*position) 1.0 0.0 2.0 bias = (position - 0.5) * contrast) 0.0 when contrast = 0.0 0.0 0.0 when contrast = 1.0 -0.5 0.5 when contrast = 2.0 -1.0 1.0 saturation = (2*position) 0.5 0.0 2.0 threshold = (2*position) - 1.0 -0.5 -1.0 1.0 The saturation and threshold values are converted to measures of contrast and bias: contrast = (saturation - threshold) bias = (saturation + threshold) / 2 When a slider changes, the {red, green, blue} intensities in the colormap are changed according to: intensity = ((intensity * contrast) + bias) ** gamma . *********** * * * Reset * * * *********** Selection of the "Reset" button causes all sliders to be returned to their default positions, the guns selection to be all three R-G-B, and the mode to be Normal. The effect is to restore the color table to its original state as determined by the default or selected lut. ************************ * * * Color Palette Menu * * * ************************ Clicking this button pops up the color map file browser, from which one may choose a alternate color map. This is equivalent to clicking the Colormap button on the Options submenu. ************* * * * Summary * * * ************* Color table adjustments may prove useful in understanding an image's pixel data. One is encouraged to try different look-up-tables and try out some of the adjustments allowed with this menu of modes, guns, and sliders. ********************* * * * Acknowledgement * * * ********************* Most of the ideas related to these color table adjustments are due to Michael VanHilst and the SAOimage software package from the Smithsonian Astronomical Observatory.