contour: Make a contour plot of an image

Package: plot

Usage

contour image

Parameters

image: Two dimensional image or image section to be contoured.

floor = INDEF: Minimum value to be contoured. If floor = INDEF, the data minimum is used for the floor.

ceiling = INDEF: Maximum value to be contoured. If ceiling = INDEF, the data maximum is used for the ceiling.

zero = 0: Greyscale value of the zero contour, i.e., the value of a zero point shift to be applied to the image data before plotting. Does not affect the values of the floor and ceiling parameters.

ncontours = 0: Number of contours to be drawn. If 0, the contour interval may be specified, otherwise 20-30 nicely spaced contours are drawn. A maximum of 40 contours can be drawn.

interval = 0: Contour interval. If 0, a contour interval is chosen which places 20 to 30 contours spanning the intensity range of the image.

nhi = -1: If -1, highs and lows are not marked. If 0, highs and lows are marked on the plot. If 1, the intensity of each pixel is marked on the plot.

dashpat = 528: Dash pattern for negative contours.

device = "stdgraph": Output device (stdgraph, stdplot, or the name of a physical device).

xres = 64, yres = 64: The input image is block averaged or subsampled to this resolution.

preserve = yes: If preserve = yes, the aspect ratio of the image is preserved when achieving the resolution specified by xres and yres.

subsample = no: The resolution specified by xres, yres is achieved by block averaging unless subsample = yes.

perimeter = yes: A crtpict perimeter is drawn around the contour plot with labeled tickmarks.

label= yes: By default, the value of each major contour is embedded in the contour line. This can be disabled by setting label=no.

vx1 = 0.0, vx2 = 0.0, vy1 = 0.0, vy2 = 0.0: The device viewport, in normalized device coordinates (from 0.0 to 1.0 inclusive). If not specified by the user, contour automatically centers the plot on the device viewport.

fill = no: Fill the output viewport regardless of the device aspect ratio?

title = "imtitle": A title to be centered above the plot. The user can specify a title string; the default string is the image title.

append = no: Append to an existing plot?

Description

Contours are traced, smoothed with splines under tension, and optionally printed with embedded intensity labels. Positive contours are printed as solid lines and negative contours as dashed lines. The plot is generated by the NCAR conrec utility, using dashsmth to smooth the contours and draw dashed lines.

To speed up the contouring, the resolution of the image to be plotted can be decreased to xres by yres. When preserve = yes, contour automatically reduces the image in both directions by the same factor, which is the larger of [ncolumns / xres or nlines / yres]. If the aspect ratio is not being preserved, the x and y dimensions are independently reduced to the specified resolution. No reduction is done if xres and yres = 0, if the input image is an image section, or if the image is smaller than xres by yres.

If the device viewport (plotting area) is not set by the user, contour automatically sets a viewport centered on the output device. The default value of fill=no means the viewport will be adjusted so that equal numbers of image pixels in x and y will occupy equal lengths when plotted. That is, when fill = no, a unity aspect ratio is enforced, and square images are represented as square plots regardless of the device aspect ratio. On devices with non square full device viewports (e.g., the vt640), a square image will appear extended when fill = yes. To completely fill the device viewport with contour lines, disable perimeter drawing and enable fill, and nothing but the contour map will be drawn.

Contour plots may be overlaid on a displayed image by setting the output device to "imd" for image display and the contouring parameters fill and perimeter to "yes" and "no" respectively. By default green contours will be drawn on the image display. Other choices for device are "imdr", "imb", "imdy", "imdw" and "imdg" for red, blue, yellow, white and green output contours respectively.

Examples

1. Draw a contour plot of a 512 square image on the graphics terminal. With the default values for xres and yres, the image would automatically be block averaged by a factor of 8 in x and y.

cl> contour crab.5009

2. The plot could be output to the plotter as a background job:

cl> contour crab.5009 device=stdplot &

3. Place a ceiling at an intensity value of 500 to cut out a noise spike. The plot has been moved to the lower left corner of the display.

cl> cont crab.5009 ceil=500 vx1=.1 vx2=.6 vy1=.1 vy2=.6

4. Overlay a contour plot of an image on the same image displayed on the display device. Note that the CONTOUR parameters fill and perimeter must be on and off respectively, the fill parameter should be specified for the DISPLAY task to ensure the image fills the frame buffer in the same way.

cl> display m51 1 fill+
cl> cont m51 fill+ per- device=imd

Time requirements

The time required for contour depends on the number of contours being drawn - that is, the size and smoothness of the intensity array. A 512 square image of "average" smoothness, with x and y resolution equal to 64, requires about 22 cpu seconds with block averaging. Using subsampling rather than block averaging, contour takes 16 seconds. A noisy picture will be plotted more quickly if block averaged rather than subsampled.

Bugs

If block averaging is used the precision with which a contour is drawn will be no better than the blocking factor. For example, if a contour map drawn with a block averaging factor of 8 is overlaid on an image of a starfield, contours drawn around stars in the field may not appear to be centered. If this is a problem the solution is to increase the plotting resolution using the xres and yres parameters.

It should be possible to have list input as well as image section input.