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display -- Load and display images in an image display


display image frame


Image to be loaded.
Display frame to be loaded.

bpmask = "BPM"
Bad pixel mask. The bad pixel mask is used to exclude bad pixels from the automatic intensity mapping algorithm. It may also be displayed as an overlay or to interpolate the input image as selected by the bpdisplay parameter. The bad pixel mask is specified by a pixel list image (.pl extension) or an regular image. Values greater than zero define the bad pixels. The special value "BPM" may be specified to select a pixel list image defined in the image header under the keyword "BPM". If the bad pixel mask cannot be found a warning is given and the bad pixel mask is not used in the display.
bpdisplay = "none" (none|overlay|interpolate)
Type of display for the bad pixel mask. The options are "none" to not display the mask, "overlay" to display as an overlay with the colors given by the bpcolors parameter, or "interpolate" to linearly interpolate across the bad pixels in the displayed image. Note that the bad is still used in the automatic intensity scaling regardless of the type of display for the bad pixel mask.
bpcolors = "red"
The mapping between bad pixel values and display colors or intensity values when the bad pixels are displayed as an overlay. There are two forms, explicit color assignments for values or ranges of values, and expressions. These is described in the OVERLAY COLOR section.

overlay = ""
Overlay mask to be displayed. The overlay mask may be a pixel list image (.pl extension) or a regular image. Overlay pixels are identified by values greater than zero. The overlay values are displayed with a mapping given by the ocolors parameter. If the overlay cannot be found a warning is given and the overlay is not displayed.
ocolors = "green"
The mapping between bad pixel values and display colors or intensity values when the bad pixels are displayed as an overlay. There are two forms, explicit color assignments for values or ranges of values, and expressions. These is described in the OVERLAY COLOR section.

erase = yes
Erase frame before loading image?
border_erase = no
Erase unfilled area of window in display frame if the whole frame is not erased?
select_frame = yes
Select the display frame to be the same as the frame being loaded?
repeat = no
Repeat the previous spatial and intensity transformations?
fill = no
Interpolate the image to fit the display window?
zscale = yes
Apply an automatic intensity mapping algorithm when loading the image?
contrast = 0.25
Contrast factor for the automatic intensity mapping algorithm. If a value of zero is given then the minimum and maximum of the intensity sample is used.
zrange = yes
If not using the automatic mapping algorithm (zscale = no ) map the full range of the image intensity to the full range of the display? If the displayed image has current min/max values defined these will be used to determine the mapping, otherwise the min/max of the intensity sample will be used. The MINMAX task can be used to update the min/max values in the image header.
zmask = ""
Pixel mask selecting the sample pixels for the automatic or range intensity mapping algorithm. The pixel mask may be a pixel list image (.pl extension), a regular image, or an image section. The sample pixels are identified by values greater than zero in the masks and by the region specified in an image section. If no mask specification is given then a uniform sample of approximately nsample good pixels will be used. The nsample parameter also limits the number of sample pixels used from a mask. Note that pixels identified by the bad pixel mask will be excluded from the sample.
nsample = (minimum of 100)
The number of pixels from the image sampled for computing the automatic intensity scaling. This number will be uniformly sampled from the image if the default zmask is used otherwise the first nsample pixels from the specified mask will be used.
xcenter = 0.5, ycenter = 0.5
Horizontal and vertical centers of the display window in normalized coordinates measured from the left and bottom respectively.
xsize = 1, ysize = 1
Horizontal and vertical sizes of the display window in normalized coordinates.
xmag = 1., ymag = 1.
Horizontal and vertical image magnifications when not filling the display window. Magnifications greater than 1 map image pixels into more than 1 display pixel and magnifications less than 1 map more than 1 image pixel into a display pixel.
order = 0
Order of the interpolator to be used for spatially interpolating the image. The current choices are 0 for pixel replication, and 1 for bilinear interpolation.
z1, z2
Minimum and maximum image intensity to be mapped to the minimum and maximum display levels. These values apply when not using the automatic or range intensity mapping methods.
ztrans = "linear"
Transformation of the image intensity levels to the display levels. The choices are:
Map the minimum and maximum image intensities linearly to the minimum and maximum display levels.
Map the minimum and maximum image intensities linearly to the range 1 to 1000, take the logarithm (base 10), and then map the logarithms to the display range.
Apply no mapping of the image intensities (regardless of the values of zcale, zrange, z1, and z2 ). For most image displays, values exceeding the maximum display value are truncated by masking the highest bits. This corresponds to applying a modulus operation to the intensity values and produces "wrap-around" in the display levels.
User supplies a look up table of intensities and their corresponding greyscale values.
lutfile = ""
Name of text file containing the look up table when ztrans = user. The table should contain two columns per line; column 1 contains the intensity, column 2 the desired greyscale output.


The specified image and overlay mask are loaded into the specified frame of the standard image display device ("stdimage"). For devices with more than one frame it is possible to load an image in a frame different than that displayed on the monitor. An option allows the loaded frame to become the displayed frame. The previous contents of the frame may be erased (which can be done very quickly on most display devices) before the image is loaded. Without erasing, the image replaces only those pixels in the frame defined by the display window and spatial mapping described below. This allows displaying more than one image in a frame. An alternate erase option erases only those pixels in the defined display window which are not occupied by the image being loaded. This is generally slower than erasing the entire frame and should be used only if a display window is smaller than the entire frame.

The image is mapped both in intensity and in space. The intensity is mapped from the image pixel values to the range of display values in the device. Spatial interpolation maps the image pixel coordinates into a part of the display frame called the display window. Many of the parameters of this task are related to these two transformations.

A bad pixel mask may be specified to be displayed as an overlay or to interpolate the displayed image. It is also used to exclude bad pixels from the automatic intensity scaling. The bad pixel mask is specified by the parameter bpmask and the display mode by the bpdisplay parameter. The overlay display option uses the bpcolors parameters to specify a color mapping as described in the OVERLAY COLOR section. Interpolation consists of linear interpolation across columns if the mask value is one, across lines if the mask value is two, or across the shortest direction for other values. This interpolation is done on the input data before any spatial interpolation and filling is done. It does not modify the input data. The task fixpix provides the same algorithm to fix the data in the image.

An overlay mask may be specified by the overlay parameter. Any value greater than zero in the overlay mask will be displayed in the color or intensity specified by the ocolor parameter (see the OVERLAY COLOR section).

Note that bad pixel masks in "pixel list" format are constrained to non-negative values. When an image is used instead of a pixel list the image is internally converted to a pixel list. Negative values are set to zero or good pixels and positive real values are truncated to the nearest integer.

A display window is defined in terms of the full frame. The lower left corner of the frame is (0, 0) and the upper right corner is (1, 1) as viewed on the monitor. The display window is specified by a center (defaulted to the center of the frame (0.5, 0.5)) and a size (defaulted to the full size of the frame, 1 by 1). The image is loaded only within the display window and does not affect data outside the window; though, of course, an initial frame erase erases the entire frame. By using different windows one may load several images in various parts of the display frame.

If the option fill is selected the image and overlay mask are spatially interpolated to fill the display window in its largest dimension (with an aspect ratio of 1:1). When the display window is not automatically filled the image is scaled by the magnification factors (which need not be the same) and centered in the display window. If the number of image pixels exceeds the number of display pixels in the window only the central portion of the image which fills the window is loaded. By default the display window is the full frame, the image is not interpolated (no filling and magnification factors of 1), and is centered in the frame. The spatial interpolation algorithm is described in the section MAGNIFY AND FILL ALGORITHM.

There are several options for mapping the pixel values to the display values. There are two steps; mapping a range of image intensities to the full display range and selecting the mapping function or transformation. The mapping transformation is set by the parameter ztrans . The most direct mapping is "none" which loads the image pixel values directly without any transformation or range mapping. Most displays only use the lowest bits resulting in a wrap-around effect for images with a range exceeding the display range. This is sometimes desirable because it produces a contoured image which is not saturated at the brightest or weakest points. This is the fastest method of loading the display. Another transformation, "linear", maps the selected image range linearly to the full display range. The logarithmic transformation, "log", maps the image range linearly between 1 and 1000 and then maps the logarithm (base 10) linearly to the full display range. In the latter transformations pixel values greater than selected maximum display intensity are set to the maximum display value and pixel values less than the minimum intensity are set to the minimum display value.

Methods for setting of the range of image pixel values, z1 and z2 , to be mapped to the full display range are arranged in a hierarchy from an automatic mapping which gives generally good result for typical astronomical images to those requiring the user to specify the mapping in detail. The automatic mapping is selected with the parameter zscale . The automatic mapping algorithm is described in the section ZSCALE ALGORITHM and has three parameters, zmask , nsample and contrast .

When ztrans = user, a look up table of intensity values and their corresponding greyscale levels is read from the file specified by the lutfile parameter. From this information, a piecewise linear look up table containing 4096 discrete values is composed. The text format table contains two columns per line; column 1 contains the intensity, column 2 the desired greyscale output. The greyscale values specified by the user must match those available on the output device. Task showcap can be used to determine the range of acceptable greyscale levels. When ztrans = user, parameters zscale , zrange and zmap are ignored.

If the zscale algorithm is not selected the zrange parameter is examined. If zrange is yes then the minimum and maximum pixel values in the image are taken from the image header or estimated from the intensity sample and z1 and z1 are set to those values, respectively. This insures that the full range of the image is displayed but is generally slower than the zscale algorithm (because all the image pixels must be examined) and, for images with a large dynamic range, will generally show only the brightest parts of the image.

Finally, if the zrange algorithm is not selected the user specifies the values of z1 and z2 directly.

Often several images are to be loaded with the same intensity and spatial transformations. The option repeat repeats the transformations from the previous image loaded.


The zscale algorithm is designed to display the image values near the median image value without the time consuming process of computing a full image histogram. This is particularly useful for astronomical images which generally have a very peaked histogram corresponding to the background sky in direct imaging or the continuum in a two dimensional spectrum.

The sample of pixels, specified by values greater than zero in the sample mask zmask or by an image section, is selected up to a maximum of nsample pixels. If a bad pixel mask is specified by the bpmask parameter then any pixels with mask values which are greater than zero are not counted in the sample. Only the first pixels up to the limit are selected where the order is by line beginning from the first line. If no mask is specified then a grid of pixels with even spacing along lines and columns that make up a number less than or equal to the maximum sample size is used.

If a contrast of zero is specified (or the zrange flag is used and the image does not have a valid minimum/maximum value) then the minimum and maximum of the sample is used for the intensity mapping range.

If the contrast is not zero the sample pixels are ranked in brightness to form the function I(i) where i is the rank of the pixel and I is its value. Generally the midpoint of this function (the median) is very near the peak of the image histogram and there is a well defined slope about the midpoint which is related to the width of the histogram. At the ends of the I(i) function there are a few very bright and dark pixels due to objects and defects in the field. To determine the slope a linear function is fit with iterative rejection;

I(i) = intercept + slope * (i - midpoint)

If more than half of the points are rejected then there is no well defined slope and the full range of the sample defines z1 and z2 . Otherwise the endpoints of the linear function are used (provided they are within the original range of the sample):

        z1 = I(midpoint) + (slope / contrast) * (1 - midpoint)
        z2 = I(midpoint) + (slope / contrast) * (npoints - midpoint)

As can be seen, the parameter contrast may be used to adjust the contrast produced by this algorithm.


The spatial interpolation algorithm magnifies (or demagnifies) the image (and the bad pixel and overlay masks) along each axis by the desired amount. The fill option is a special case of magnification in that the magnification factors are set by the requirement that the image just fit the display window in its maximum dimension with an aspect ratio (ratio of magnifications) of 1. There are two requirements on the interpolation algorithm; all the image pixels must contribute to the interpolated image and the interpolation must be time efficient. The second requirement means that simple linear interpolation is used. If more complex interpolation is desired then tasks in the IMAGES package must be used to first interpolate the image to the desired size before loading the display frame.

If the magnification factors are greater than 0.5 (sampling step size less than 2) then the image is simply interpolated. However, if the magnification factors are less than 0.5 (sampling step size greater than 2) the image is first block averaged by the smallest amount such that magnification in the reduced image is again greater than 0.5. Then the reduced image is interpolated to achieve the desired magnifications. The reason for block averaging rather than simply interpolating with a step size greater than 2 is the requirement that all of the image pixels contribute to the displayed image. If this is not desired then the user can explicitly subsample using image sections. The effective difference is that with subsampling the pixel-to-pixel noise is unchanged and small features may be lost due to the subsampling. With block averaging pixel-to-pixel noise is reduced and small scale features still contribute to the displayed image.


The masks specified by the bpmask and overlay parameters may be displayed as color overlays on the image data. The non-zero pixels in the mask are assigned integer display values. The values may fall in the same range, 1 to 200, as the mapped image pixel data values and will behave the same way as the pixel values when the display map is interactively adjusted. Values of 0 and 201 to 255 may be used and depend on the display server and display resource definitions. The expected or standard server behavior is that 0 is the background color and 201 to 255 are various colors with the lower numbers being the more standard primary colors. The expected colors are:

        Value   Color               Value   Color
        201     white (cursor)      210     coral
        202     black (background)  211     maroon
        203     white               212     orange
        204     red                 213     khaki
        205     green               214     orchid
        206     blue                215     turquoise
        207     yellow              216     violet
        208     cyan                217     wheat
        209     magenta

The values 201 and 202 are tied to the cursor and background resource colors. These are generally white and black respectively. Values above 217 are not defined and depend on the current state of the color table for the window system.

The mapping between between mask values and overlay colors are specified by the bpcolors and ocolors parameters. There are two mapping syntax, a list and an expression.

The list syntax consists of a comma delimited set of values and assignments with one of the following forms.


where color may be a color name, a color value, or value to be added or subtracted to the mask value to yield a color value. Color names may be black, white, red, green, blue, yellow, cyan, magenta, or transparent with case ignored and abbreviations allowed. Transparent does the obvious of being invisible. These values are based on the default resource colors for the display servers (as shown above) and any custom definitions may result in incorrect colors.

The color values are unsigned integers (no + or -) or values to be added or subtracted are given as signed integers. The first form provides the default intensity or color for all mask values. Note that if no default color is specified the default will be white. The other forms map a mask value or range of mask values to a color. In a list the last color defined for the default or mask value will be used.

The addition or subtraction from mask values provides a mechanism to have the bad pixel or overlay masks encode a variety of overlay colors. Note that to display the mask values directly as colors one would use the color value "+0". Subtraction may produce values less than zero which then are not visible; i.e. equivalent to "transparent".

The following examples illustrate the features of the syntax.

    ocolors=""          Display in default white
    ocolors="red"       Display in red
    ocolors="+0"        Display mask values as color values
    ocolors="+200"      Display mask values offset by 200


The last example has a default color of 205, mask values of 1 are red, mask values of 2 are yellow, mask values of 10 to 20 are cyan, and mask values of 30 to 40 are displayed as intensities 130 to 140.

Expressions are identified by being enclosed in parentheses. This uses the general IRAF expression syntax (see expressions ). The mask values are referenced by the character $. The same named colors (black, white, red, green, blue, yellow, cyan, magenta, and transparent) may be used in place of color values. Expressions must evaluate to integer values. To avoid needing special handling of input mask values of zero, all pixels with input mask values of zero are not shown regardless of the expression value.

There are currently two function extensions, "colors" and "acenum". In both functions the first and only required argument, arg1, is an integer value. Typically this will $ or a function based on $.

The "colors" function maps input values with a modulus type behavior. The optional second argument, arg2, is a color value for mapping zero. As noted above, if the input mask value is zero it will not be displayed. However, functions applied to non-zero input mask values may return a value of zero which may then be displayed with the specified color. The default is transparent. The next two optional arguments (arg3 and arg4) define a color range with defaults of 204 to 217. If only arg3 is specified then arg4 takes the value of arg3, thus having the effect of a constant output color. Positive values of the first argument are mapped to a color value by

    if arg1 is 0:       result = arg2
    if arg1 greater 0:  result = arg3 + mod ($-1, arg4-arg3+1)
    otherwise:          result = arg1

This function is primarily used to make colorful displays of regions defined with different mask values.

The "acenum" function handles ace package object detection masks which include bit flags. Each object in the mask has an object number with value greater than 10. Values less than 10 are passed along during detection and generally identify detector or saturated bad pixels. Along with the the object number there may be zero or more bit flags set. This function removes the bit flags and returns the mask number. The optional second argument, arg2, is a string of letters which selects pixels with certain sets of bit flags. The bit flags are:

    B -- a bad pixel treated as a good for detection
    D -- original detection (i.e. without G or S flag)
    E -- edge pixel used for displaying detection isophotes
    F -- object contains a bad pixel
    G -- grown pixel
    S -- pixel not assigned to an object during splitting

The default of arg2 is "BDEG" which essentially returns all pixels in an object.

The acenum function also returns 0 for the pixels with values between one and ten and -1 for the pixels not selected by the flags. The value of zero may be made visible using the colors function. The two functions are often used in concert:


Note that when filling and anti-aliasing the behavior of the overlay colors may be different than intended.


For the purpose of these examples we assume a display with four frames, 512 x 512 in size, and a display range of 0 to 255. Also consider two images, image1 is 100 x 200 with a range 200 to 2000 and image2 is 2000 x 1000 with a range -1000 to 1000. To load the images with the default parameters:

        cl> display image1 1
        cl> display image2 2

The image frames are first erased and image1 is loaded in the center of display frame 1 without spatial interpolation and with the automatic intensity mapping. Only the central 512x512 area of image2 is loaded in display frame 2

To load the display without any intensity transformation:

cl> cvl image1 1 ztrans=none

The next example interpolates image2 to fill the full 512 horizontal range of the frame and maps the full image range into the display range. Note that the spatial interpolation first block averages by a factor of 2 and then magnifies by 0.512.

cl> display image2 3 fill+ zscale-

The next example makes image1 square and sets the intensity range explicitly.

cl> display image1 4 zscale- zrange- z1=800 z2=1200 xmag=2

The next example loads the two images in the same frame side-by-side.

        cl> display.xsize=0.5
        cl> display image1 fill+ xcen=0.25
        cl> display image2 erase- fill+ xcen=0.75


The bad pixel mask, overlay mask, sample mask, and overlay colors parameters and functionality have been added. The "nsample_lines" parameter is now an "nsample" parameter.

Bugs in the coordinate system sent to the image display for cursor readback were fixed.


The "repeat" option is not implemented.


cvl, magnify, implot, minmax, fixpix

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