The MAP_IMAGE function warps an image (or other dataset) to the current map projection. This function provides an easy method for displaying geographical data as an image on a map. The MAP_SET procedure should be called prior to calling MAP_IMAGE.
MAP_IMAGE works in image (graphic) space. For each destination pixel (when COMPRESS is set to one) MAP_IMAGE calculates the latitude and longitude by applying the inverse map projection. This latitude and longitude are then used to index and interpolate the Image argument, obtaining an interpolated value for the destination pixel. The time required by MAP_IMAGE depends mainly on the number of pixels in the destination and the setting of the COMPRESS parameter.
| Note |
Result = MAP_IMAGE( Image [, Startx, Starty [, Xsize, Ysize]] [, LATMIN=degrees{-90 to 90}] [, LATMAX=degrees{-90 to 90}]
[, LONMIN=degrees{-180 to 180}] [, LONMAX=degrees{-180 to 180}] [, /BILINEAR] [, COMPRESS=value] [, SCALE=value] [, MAX_VALUE=value] [, MIN_VALUE=value] [, MISSING=value] )
Returns the image or dataset warped to the current map projection.
A two-dimensional array containing the image to be overlaid on the map.
A named variable that, upon return, contains the X coordinate position where the left edge of the image should be placed on the screen.
A named variable that, upon return, contains the Y coordinate position where the left edge of the image should be placed on the screen.
A named variable that, upon return, contains the width of the image expressed in graphic coordinate units. If the current graphics device uses scalable pixels, the values of Xsize and Ysize should be passed to the TV procedure.
A named variable that, upon return, contains the height of the image expressed in graphic coordinate units. If the current graphics device uses scalable pixels, the values of Xsize and Ysize should be passed to the TV procedure.
The latitude corresponding to the first row of Image. The default is -90 degrees. Note also that -90° £ LATMIN < LATMAX £ 90°.
The latitude corresponding to the last row of Image. The default value is 90 degrees. Note also that -90° £ LATMIN < LATMAX £ 90°.
The longitude corresponding to the first (leftmost) column of the Image argument. Select LONMIN so that -180° £ LONMIN £ 180°. The default value is -180.
The longitude corresponding to the last (rightmost) column of the Image argument. Select LONMAX so that it is larger than LONMIN. If the longitude of the last column is equal to (LONMIN - (360. /Nx)) MODULO 360, it is assumed that the image covers all longitudes (Nx being the total number of columns in the Image argument).
Set this flag to use bilinear interpolation to soften edges in the returned image, otherwise, nearest neighbor sampling is used.
This keyword, the interpolation compression flag, controls the accuracy of the results from MAP_IMAGE. The default is 4 for output devices with fixed pixel sizes. The inverse projection transformation is applied to each ith row and column. Setting this keyword to a higher number saves time while lower numbers produce more accurate results. Setting this keyword to 1 solves the inverse map transformation for every pixel of the output image.
Set this keyword to the pixel/graphics scale factor for devices with scalable pixels (e.g., PostScript). The default is 0.02 pixels/graphic coordinate. This setting yields an approximate output image size of 350 x 250. Make this number larger for more resolution (and larger PostScript files and images), or smaller for faster, smaller, and less accurate images.
Data points with values equal to or greater than this value will be treated as missing data, and will be set to the value specified by the MISSING keyword.
Data points with values equal to or less than this value will be treated as missing data, and will be set to the value specified by the MISSING keyword.
The pixel value to set areas outside the valid map coordinates. If this keyword is omitted, areas outside the map are set to 255 (white) if the current graphics device is PostScript, otherwise they are set to 0.
The following lines of code set up an orthographic map projection and warp a simple image to it.
; Create a simple image to be warped: image = BYTSCL(SIN(DIST(400)/10)) ; Display the image so we can see what it looks like before ; warping: TV, image latmin = -65 latmax = 65 ; Left edge is 160 East: lonmin = 160 ; Right edge is 70 West = +360: lonmax = -70 + 360 MAP_SET, 0, -140, /ORTHOGRAPHIC, /ISOTROPIC, $ LIMIT=[latmin, lonmin, latmax, lonmax] result = MAP_IMAGE(image,Startx,Starty, COMPRESS=1, $ LATMIN=latmin, LONMIN=lonmin, $ LATMAX=latmax, LONMAX=lonmax) ; Display the warped image on the map at the proper position: TV, result, Startx, Starty ; Draw gridlines over the map and image: MAP_GRID, latdel=10, londel=10, /LABEL, /HORIZON ; Draw continent outlines: MAP_CONTINENTS, /coasts
Introduced: Pre 4.0
MAP_CONTINENTS, MAP_GRID, MAP_PATCH, MAP_SET