This function extracts single components of the pixel information from the topmost window.
Lower left corner and size of desired region.
Whether to bring window to top before reading.
The possible components are
"luminance" (the sum of the three
colors). All are scaled from 0 to 1.
Note that the luminance is kept below 1 by truncating the sum; this is the
definition used for the
GL_LUMINANCE component in OpenGL.
A vector, matrix or array containing the desired components. If one component is requested, a vector or matrix will be returned depending on the size of block requested (length 1 dimensions are dropped); if more, an array, whose last dimension is the list of components.
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