equ_histo_imageequ_histo_imageEquHistoImageEquHistoImageequ_histo_image (Operator)

Name

equ_histo_imageequ_histo_imageEquHistoImageEquHistoImageequ_histo_image — Histogram linearization of images

Signature

Description

The operator equ_histo_imageequ_histo_imageEquHistoImageEquHistoImageequ_histo_image enhances the contrast. The starting point is the histogram of the input images. The following simple gray value transformation f(g) is carried out for byte images: h(x) describes the relative frequency of the occurrence of the gray value x. For uint2 images, the only difference is that the value 255 is replaced with a different maximum value. The maximum value is computed from the number of significant bits stored with the input image, provided that this value is set. If not, the value of the system parameter 'int2_bits'"int2_bits""int2_bits""int2_bits""int2_bits" is used (see set_systemset_systemSetSystemSetSystemset_system), if this value is set (i.e., different from -1). If none of the two values is set, the number of significant bits is set to 16.

This transformation linearizes the cumulative histogram. Maxima in the original histogram are “spreaded” and thus the contrast in image regions with these frequently occurring gray values is increased. Supposedly homogeneous regions receive more easily visible structures. On the other hand, of course, the noise in the image increases correspondingly. Minima in the original histogram are dually “compressed”. The transformed histogram contains gaps, but the remaining gray values used occur approximately at the same frequency (“histogram equalization”).

Attention

The operator equ_histo_imageequ_histo_imageEquHistoImageEquHistoImageequ_histo_image primarily serves for optical processing of images for a human viewer. For example, the (local) contrast spreading can lead to a detection of fictitious edges.

Note that filter operators may return unexpected results if an image with a reduced domain is used as input. Please refer to the chapter Filters.

Execution Information

• Multithreading type: reentrant (runs in parallel with non-exclusive operators).
• Multithreading scope: global (may be called from any thread).
• Automatically parallelized on tuple level.
• Automatically parallelized on channel level.

Parameters

Possible Successors

disp_imagedisp_imageDispImageDispImagedisp_image

Alternatives

equ_histo_image_rectequ_histo_image_rectEquHistoImageRectEquHistoImageRectequ_histo_image_rect, scale_imagescale_imageScaleImageScaleImagescale_image, scale_image_maxscale_image_maxScaleImageMaxScaleImageMaxscale_image_max, illuminateilluminateIlluminateIlluminateilluminate

See also

equ_histo_image_rectequ_histo_image_rectEquHistoImageRectEquHistoImageRectequ_histo_image_rect, scale_imagescale_imageScaleImageScaleImagescale_image

References

R.C. Gonzales, P. Wintz: “Digital Image Processing”; Second edition; Addison Wesley; 1987.

Module

Foundation