crtpict: Generate greyscale plots of IRAF images

Package: plot

Usage

crtpict input

Parameters

input: Input images to be processed.

device = "dicomed": The output device.

auto_fill = yes: If set to yes, the image will be scaled to fit the device viewport. The aspect ratio is always preserved when auto_fill = yes.

xmag = 1.0, ymag = 1.0: When auto_fill = no, the x and y magnification ratios are specified by these parameters.

replicate = yes: The image pixels are block replicated to fit the device viewport when replicate = yes. Otherwise, the pixels are linearly interpolated to match the device pixels.

x_block_avg = 1, y_block_avg = 1: These parameters are used when replicate = no to decrease the effective output device resolution, and speed up the interpolation. The pixels are interpolated to the block averaged output device, then block replicated to fill the device viewport.

ztrans = "auto"

This parameter specifies how the image intensities are mapped into the greyscale values of the output device. Intensity z1 maps to black, z2 to white. The 4 choices for ztrans are:

"auto"          - z1 and z2 centered on median of image
"min_max"       - set z1 and z2 to specified intensities
"none"          - truncate intensities to fit output range
"user"          - user supplies look up table of 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.

contrast = 0.25: Used when automatically determining z1 and z2. The slope of the transfer function is divided by contrast, so negative values of contrast result in a negative transfer function.

nsample_lines = 25: Used when automatically determining z1 and z2, this parameter sets the number of image lines to be sampled when determining the median.

z1 = 0.0, z2 = 0.0: These parameters are used when ztrans = "min_max", to specify which pixel values map to black and white.

perimeter = yes: Draw annotated axes around the plot perimeter?

image_fraction = 0.70: The fraction of the vertical device viewport reserved for the image.

graphics_fraction = 0.20: The fraction of the vertical device viewport reserved for histogram plots and id information.

greyscale_fraction = 0.05: The fraction of the vertical device viewport reserved for the greyscale step wedge.

output = "": Output metacode is appended to this file. By naming an output file, the metacode can be "trapped", and the normal spooling process intercepted.

Description

Procedure crtpict makes a photographic hardcopy plot of IRAF images.

The image can be automatically scaled to fill the output plotting window, with the aspect ratio preserved, by setting auto_fill = yes. When auto_fill = no, magnification factors for the axes are entered as xmag and ymag, where negative values (as well as fractional values < 1.0), indicate that the image is to be reduced. By default, the imaged is enlarged by block replication. By setting replicate = no, the image will be linearly interpolated to fit the device area. (In this case, to speed things up, the block_avg parameters can be set to reduce the effective output resolution.) In either case, if an image needs to be reduced in size, it will be decimated.

Four methods of determining the greyscale transformation are available. When ztrans = "none", no transformation between intensity and greyscale level occurs, the intensities are simply copied, which will most likely result in truncation. With this method, the lowest bits of each pixel, the lowest level variations, are always shown, regardless of the dynamic range of the image.

When ztrans = "auto", the greyscale levels are automatically centered on the median of the image pixels. The window of intensities spanned by the greyscale is controlled by parameter contrast, which is divided into the calculated slope of the transfer function. The larger the absolute value of contrast, the higher the contrast in the output image. A subset of the image pixels are used to determine the median; the number of lines sampled is nsample_lines.

When ztrans = "min_max", intensity z1 maps to the minimum greyscale level (black), z2 maps to the maximum greyscale level (white) and the transfer function is linear in between these two endpoints. If z1 = z2, the image min and max map to black and white, modified by contrast. (NOTE: When running crtpict on an image created with snap, ztrans should be set to "min_max", with z1 = 0 and z2 = 1023, the maximum output value possible from the IIS.)

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, crtpict constructs a piecewise linear look up table containing 4096 discrete values. 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.

Examples

1. To subsample every 4th pixel of a large image, fill the output area and use previously determined values of z1 and z2 for the greyscale transformation the command would be:

cl> crtpict sunpic[*:4,*:4] ztrans=min z1=0 z2=800

2. To process every image with the root name ccdpic, using default values of all parameters, the command would be:

cl> crtpict ccdpic*

3. To process images created with snap, ztrans and z2 must be changed from their default values:

cl> crtpict iis.snap ztrans=min z2=1023

4. Image `mypic' is processed using the look up table in file `mylut',

cl> crtpict mypic ztrans=user lutfile=mylut

Where file `mylut' contains this information:

10      40
1500    100
2500    100
3500    200
7500    255

Timing

For a 512 x 512 real image, crtpict takes about 40 cpu seconds with auto_fill and replicate = yes. When auto_fill = yes but replicate = no, crtpict requires almost 400 cpu seconds.