glHistogram
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## Name

glHistogram — define histogram table

## C Specification

`void `glHistogram`` | `(` | `GLenum `target`, GLsizei `width`, GLenum `internalformat`, GLboolean `sink`);`

## Python Specification

``glHistogram`` | `(` | ``target`, `width`, `internalformat`, `sink`) → `None``

## Parameters

`target`
  The histogram whose parameters are to be set. Must be one of `GL_HISTOGRAM` or `GL_PROXY_HISTOGRAM`.
`width`
  The number of entries in the histogram table. Must be a power of 2.
`internalformat`
  The of entries in the histogram table. Must be one of `GL_ALPHA`, `GL_ALPHA4`, `GL_ALPHA8`, `GL_ALPHA12`, `GL_ALPHA16`, `GL_LUMINANCE`, `GL_LUMINANCE4`, `GL_LUMINANCE8`, `GL_LUMINANCE12`, `GL_LUMINANCE16`, `GL_LUMINANCE_ALPHA`, `GL_LUMINANCE4_ALPHA4`, `GL_LUMINANCE6_ALPHA2`, `GL_LUMINANCE8_ALPHA8`, `GL_LUMINANCE12_ALPHA4`, `GL_LUMINANCE12_ALPHA12`, `GL_LUMINANCE16_ALPHA16`, `GL_R3_G3_B2`, `GL_RGB`, `GL_RGB4`, `GL_RGB5`, `GL_RGB8`, `GL_RGB10`, `GL_RGB12`, `GL_RGB16`, `GL_RGBA`, `GL_RGBA2`, `GL_RGBA4`, `GL_RGB5_A1`, `GL_RGBA8`, `GL_RGB10_A2`, `GL_RGBA12`, or `GL_RGBA16`.
`sink`
  If `GL_TRUE`, pixels will be consumed by the histogramming process and no drawing or texture loading will take place. If `GL_FALSE`, pixels will proceed to the minmax process after histogramming.

## Description

When `GL_HISTOGRAM` is enabled, RGBA color components are converted to histogram table indices by clamping to the range [0,1], multiplying by the width of the histogram table, and rounding to the nearest integer. The table entries selected by the RGBA indices are then incremented. (If the internal of the histogram table includes luminance, then the index derived from the R color component determines the luminance table entry to be incremented.) If a histogram table entry is incremented beyond its maximum value, then its value becomes undefined. (This is not an error.)

Histogramming is performed only for RGBA pixels (though these may be specified originally as color indices and converted to RGBA by index table lookup). Histogramming is enabled with `glEnable` and disabled with `glDisable`.

When `target` is `GL_HISTOGRAM`, `glHistogram` redefines the current histogram table to have `width` entries of the specified by `internalformat`. The entries are indexed 0 through width - 1 , and all entries are initialized to zero. The values in the previous histogram table, if any, are lost. If `sink` is `GL_TRUE`, then pixels are discarded after histogramming; no further processing of the pixels takes place, and no drawing, texture loading, or pixel readback will result.

When `target` is `GL_PROXY_HISTOGRAM`, `glHistogram` computes all state information as if the histogram table were to be redefined, but does not actually define the new table. If the requested histogram table is too large to be supported, then the state information will be set to zero. This provides a way to determine if a histogram table with the given parameters can be supported.

## Notes

`glHistogram` is present only if `GL_ARB_imaging` is returned when `glGetString` is called with an argument of `GL_EXTENSIONS`.

## Errors

`GL_INVALID_ENUM` is generated if `target` is not one of the allowable values.

`GL_INVALID_VALUE` is generated if `width` is less than zero or is not a power of 2.

`GL_INVALID_ENUM` is generated if `internalformat` is not one of the allowable values.

`GL_TABLE_TOO_LARGE` is generated if `target` is `GL_HISTOGRAM` and the histogram table specified is too large for the implementation.

`GL_INVALID_OPERATION` is generated if `glHistogram` is executed between the execution of `glBegin` and the corresponding execution of `glEnd`.

## Associated Gets

`glGetHistogramParameter`

## See Also

glGetHistogram, glResetHistogram

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