image.getchunk - Function

Get the pixel values from a regular region of the image into a Glish array

Synopsis
getchunk(blc, trc, inc, axes, list, dropdeg, getmask)

Arguments

blc	in	Bottom-Left-Corner (beginning) of pixel section
		Allowed:	Vector of integers
		Default:	Start of image
trc	in	Top-Right-Corner (end) of pixel section
		Allowed:	Vector of positive integers
		Default:	End of image
inc	in	increment (stride) along axes
		Allowed:	Vector of positive integers
		Default:	1
axes	in	Axes to average over
		Allowed:	Vector of positive integers
		Default:	None
list	in	List bounding box to logger ?
		Allowed:	T or F
		Default:	F
dropdeg	in	Drop degenerate axes ?
		Allowed:	T or F
		Default:	F
getmask	in	Get the pixel mask as well
		Allowed:	T or F
		Default:	F

Returns
Glish array, Record, or fail

Description

This function returns the pixels (and opionally the pixel mask) from the image file between blc and trc inclusively. An increment may be specified with inc. Note that if you retrieve too many pixels, you might cause swapping since the pixels are kept in memory.

Any illegal blc values are set to unity. Any illegal trc values are set to the end of the image. If any trc < blc, you get the whole image for that axis. Any illegal inc values are set to unity.

The argument axes can be used to reduce the dimensionality of the output array. It specifies which pixel axes of the image average the data over. For example, consider a 3-D image. With axes=[1,2] and all other arguments left at their defaults, the result would be a 1-D vector, a profile along the third axis, with the data averaged over the first two axes.

A related function is getregion which retrieves the pixels and pixel mask from a potentially more complex region-of-interest. Function getchunk is retained because it is faster and therefore preferable for repeated operation in loops if the pixel mask is not required and the region is a simple box.

If you ask for the pixel mask as well, the return value is a record holding two fields, 'pixel' and 'pixelmask'. If you do not ask for the pixel mask (default), the pixels are returned directly.

Example
Suppose that we have a 3-dimensional image called im. Then:

- pix := im.getchunk()                      # all pixels
- pix := im.getchunk([1,1,1], [10,10,1])    # 10 by 10 section of plane # 1
- pix := im.getchunk([1,1], [1,1])          # first spectrum
- pix := im.getchunk(inc=[1,5])             # all planes, decimated by 5 in y
#
- rec := im.getchunk(getmask=T)             # Get pixels and pixelmask
- print field_names(rec)                    # into named fields of record
pixel pixelmask

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	Package	general
	Module	images
	Tool	image