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## Specifying Pixels in an Image

In some commands it is necessary to specify the columns and rows of an image to which that command should refer.
This is done in the following way for e.g. a 2-dim frame: frame[x1,y1:x2,y2]
where the column specification, x or the row specification, y can be any of

• world coordinates, indicated via real or integer numbers:  20.0,300
• pixel numbers, indicated via integers preceded by @:  @35,@200
• or a special symbol to indicate start (<), or end (>) of a row or column; thus [@20,<:@20,>] specifies the complete 20th column of a 2-dimensional image

World coordinates are the physically meaningful coordinates with units such as wavelengths or arc seconds (which are defined in the descriptor CUNIT). Pixel numbers (starting with 1 for each dimension) are the indices of an image seen as an array.
For example, extracting the complete 12th plane from the 3-dim image stored in cube.bdf is done via
EXTRACT/IMAGE plane12 = cube [<,<,@12:>,>,@12]
Single pixels can also be written in a more direct way, via
frame[x,y,z] = value, e.g. to set the pixel in row 27 and column 1023 of the 2-dim image lola.bdf to 1.234, use lola[@1023,@27] = 1.234
The value can also be an expression made up of constants and elements of any MIDAS data structure, see the subsection 3.6.2.

Some of the commands dealing with images are:
READ/IMAGE, WRITE/IMAGE, COMPUTE/IMAGE, STATIST/IMAGE, DELETE/IMAGE, COPY/II.

Next: Specifying Sub-Image Up: MIDAS Data Structures Previous: Specifying Elements in a
http://www.eso.org/midas/midas-support.html
1999-06-09