package: Guide to color image display
Package: color
INTRODUCTION
This guide describes techniques for taking three monochrome IRAF images, a "red" image, a "green" image, and a "blue" image and making color composites. There are many techniques which depend on different hardware and software. This guide currently discusses three methods for display on an 8-bit color workstation, using Sun 24-bit RGB rasterfiles, creating a special color map and image which samples the RGB color space, and pixel dithering. The rasterfiles may be displayed or printed using a variety or non-IRAF tools which are readily available and which can be used with 8-bit workstations. The special color map and pixel dithering methods use only IRAF images and the standard SAOimage/IMTOOL display servers to display on 8-bit color workstation. These techniques are intended to provide a rudimentary color composite capability in absence of better hardware such as IIS/IVAS devices or 24-bit workstations.
For further information on the tasks described here see the approriate help pages.
SUN 24-BIT RGB RASTERFILES
The task rgbsun takes three input IRAF images and produces a 24-bit Sun rasterfile. Though this file type was developed by Sun Microsystems it is a relatively simple format which may useful on other machines having software designed to use it. The color image may be display with a variety of tools such as xv (a very powerful and generic viewer for X-window systems), xloadimage (another X-window display tool), screenload (a simple displayer on Sun computers), and snapshot (an Open-Look tool). Also some color printers can be used with this format such as a Shinko color printer.
The recommended display tool is xv which provides a great deal of capability in adjusting the color map and size. This program compresses the 24-bit colors to 8-bits on an 8-bit workstation using color dithering techniques (there is a choice of a slow and fast method). This program also provides the capability to write the picture out to other formats and one may also use screen capture tools such as xwd or snapshot to extract and possibly print the picture.
For hardcopy there is always the option of photographing the workstation screen. Different sites may also have color printers which accept the rasterfile directly or some other form of capture from the screen. At NOAO there is a Shinko color printer which may be used directly with the rasterfile to make moderate quality color prints and slides.
24-BIT to 8-BIT COLOR MAP COMPRESSION
The task rgbto8 produces an 8-bit color map which samples the full range of RGB color values and an associated image with values indexing the color map. The compression algorithm is called the Median Cut Algorithm and the image is dithered with this color map using the Floyd-Steinberg algorithm. The resulting image is a short image with 199 values. The color map is output in either a format suitable for use with SAOimage or with IMTOOL. This method is recommended over the pixel dithering method.
The RGB values are input as three IRAF images. The images must each be scaled to an 8 bit range. This is done by specifying a range of input values to be mapped to the 8 bit range. In addition the range can be mapped logarithmically to allow a greater dynamic range.
The output image is displayed with rgbdisplay and SAOimage, IMTOOL, or XIMTOOL. Note that this requires V1.07 of SAOimage. The color map produced by the rgbto8 for a particular image must also be loaded into the display server manually. With IMTOOL use the setup panel and set the file name in the user1 or user2 field and then select the appropriate map. With SAOimage you select the "color" main menu function, and then the "cmap" submenu function, and then the "read" button. Note that usually a full pathname is required since the server is usually started from the login directory. For XIMTOOL the "XImtool*cmapDir1" resource must be set to the directory containing the color map and XIMTOOL must be restarted to cause the directory to be searched for color map files.
The display server must be setup in it's default contrast mapping (with IMTOOL you can use the RESET option, with XIMTOOL the "normalize" option is used, and with SAOimage you must restart) and the contrast mapping must not be changed. There are no adjustments that can be made in IMTOOL or XIMTOOL but with SAOimage you can adjust the colors using the "gamma" selections and the mouse.
8-BIT PIXEL DITHERING
1. Theory
The pixel dithering technique takes the three input IRAF images and makes a special output IRAF image in which each pixel in the input images is expanded into nine pixels in the output image with a specified pattern such as the default of
brg
r + g + b = gbr
rgb
where r is the red image pixel, g is the green image pixel, and b is the blue image pixel.
The pixel intensities are linearly mapped from a specified input range to one of three sets of 85 levels. The red pixels map to the values 0 to 84, the green pixels to the range 85 to 169, and the blue pixels to the range 170 to 254. The display server then uses a special 8-bit look up table that maps each set of 85 levels in each pure color from off to the maximum intensity. The displayed image counts on the nearby grouping of pure colors to blend in the detector, such as the eye, to give a color composite effect.
This is essentially the same technique used in some kinds of color printing and CRT monitors where each resolution element has three color phosphors and three guns to excite them. The pixel dithering is also related to black and white half-toning. As with any of these, if the image is magnified or viewed with enough resolution (by looking very closely at the display) the individual color elements can be distinguished. However, when viewed normally without magnification the effect is reasonably good.
8-BIT PIXEL DITHERING: Usage
The composite image is created by the task rgbdither and displayed with the task rgbdisplay. Unlike the display task there is no automated way to define the display ranges for the three images. These must be specified explicitly with the image is created. The ranges may be determined in a variety of ways such as by looking at the histograms, imhist, the statistics of the image, imstat, or possibly the display range produced by display. Note, however, that often the ranges used to stretch an individual image are not appropriate for color balancing between the three images.
Because each input pixel is expanded into nine pixels in the composite image the composite image will have dimensions three times larger than the input image. The blkavg parameter allows block averaging the input images at the same time that the composite image is created. If a value of 3, the default, is used then the final displayed image will have dimensions nearly the same as the input images. This is often satisfactory and one should try this first.
If one wants to display images which have a large dyanmic range it may be desirable to first take the logarithm of each image. This may be done with the logmap parameter. Other types of stretching may be accomplished by modifying the individual images first, say with imfunction.
In addition to creating and loading the composite image within IRAF it is also necessary to adjust the image display server. Either SAOimage or IMTOOL may be used. SAOimage is prefered because it is possible to make some adjustments in the color mapping while with IMTOOL one must modify the composite image by varying the z1 and z2 values for the three images.
The display servers must be set so that there is no contrast stretching. This is how the programs start initially but it may be difficult to return to this state if you adjust the contrast with the right mouse button in IMTOOL or the contrast adjustments in the (COLOR) menu of SAOimage.
You must first determine where the special color maps are located. Since the display servers are host programs they require host pathnames. You can determine the host pathname from within IRAF using the command
cl> path colorlib$saorgb.lut
puppis!/ursa/iraf/extern/color/lib/saorgb.lut
or
cl> path colorlib$imtoolrgb.lut
puppis!/ursa/iraf/extern/color/lib/imtoolrgb.lut
You can either remember these names (without the node prefix) or more simply copy the one you need to your IRAF home directory (or any place else you like) with the command
cl> copy colorlib$saorgb.lut home$
or
cl> copy colorlib$imtoolrgb.lut home$
With SAOimage load the special look up table by entering the (COLOR) menu, then the (CMAP) menu, and then pushing the (READ) button. When you are prompted for the map enter the pathname for the file saorgb.lut. For IMTOOL you need to call up the setup menu and set the pathname for the file imtoolrgb.lut in either of the user look up tables and then select the appropriate map.
For IMTOOL that is all you can do. Beware, don't adjust the contrast (the right mouse button) since this destroys the mapping between the composite image values and the look up table.
In SAOimage there are a couple of things you can do to make adjustments to the display. Bring up the color editor by clicking on the color bar. Even if you don't adjust the look up table this can be instructive. If you select (GAMMA) in the (COLOR) menu you can then move the mouse with a button down and vary the linearity of the color maps. This can be seen in the color editor. You can also adjust the individual colors by clicking the left (red), middle (green), or right (blue) buttons to either move the shown points or add and move points in the middle. Note that the abrupt discontinuity between the colors can cause sudden jumps in the color map if one point is moved past the other but you can recover by bring the point slowly back. If the map gets too messed up you can always reload the color map.
One might expect that making a hardcopy of the display would produce a comparable quality image. This may be the case by photographing the CRT screen. However, experiments with capturing the displayed image to a rasterfile and printing it on a SHINKO color printer does not produce useful hardcopy.