1991-2006
Silicon Graphics, Inc.
glCopyConvolutionFilter2D
3G

# glCopyConvolutionFilter2D

copy pixels into a two-dimensional convolution filter

## C Specification

                    void glCopyConvolutionFilter2D
                    GLenum target
                    GLenum internalformat
                    GLint x
                    GLint y
                    GLsizei width
                    GLsizei height

## Parameters

`target`

  Must be `GL_CONVOLUTION_2D`.

`internalformat`

  The internal format of the convolution filter kernel. The allowable values are `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_INTENSITY`, `GL_INTENSITY4`, `GL_INTENSITY8`, `GL_INTENSITY12`, `GL_INTENSITY16`, `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`.

`x`
`y`

  The window space coordinates of the lower-left coordinate of the pixel array to copy.

`width`

  The width of the pixel array to copy.

`height`

  The height of the pixel array to copy.

## Description

`glCopyConvolutionFilter2D` defines a two-dimensional convolution filter kernel with pixels from the current `GL_READ_BUFFER` (rather than from main memory, as is the case for glConvolutionFilter2D).

The screen-aligned pixel rectangle with lower-left corner at (`x`,\ `y`), width `width` and height `height` is used to define the convolution filter. If any pixels within this region are outside the window that is associated with the GL context, the values obtained for those pixels are undefined.

The pixels in the rectangle are processed exactly as if glReadPixels had been called with format set to RGBA, but the process stops just before final conversion. The R, G, B, and A components of each pixel are next scaled by the four 2D `GL_CONVOLUTION_FILTER_SCALE` parameters and biased by the four 2D `GL_CONVOLUTION_FILTER_BIAS` parameters. (The scale and bias parameters are set by glConvolutionParameter using the `GL_CONVOLUTION_2D` target and the names `GL_CONVOLUTION_FILTER_SCALE` and `GL_CONVOLUTION_FILTER_BIAS`. The parameters themselves are vectors of four values that are applied to red, green, blue, and alpha, in that order.) The R, G, B, and A values are not clamped to [0,1] at any time during this process.

Each pixel is then converted to the internal format specified by `internalformat`. This conversion simply maps the component values of the pixel (R, G, B, and A) to the values included in the internal format (red, green, blue, alpha, luminance, and intensity). The mapping is as follows:

Internal Format | Red | Green | Blue | Alpha | Luminance | Intensity
--- | --- | --- | --- | --- | --- | ---
`GL_ALPHA` |  |  |  | A |  |
`GL_LUMINANCE` |  |  |  |  | R |
`GL_LUMINANCE_ALPHA` |  |  |  | A | R |
`GL_INTENSITY` |  |  |  |  |  | R
`GL_RGB` | R | G | B |  |  |
`GL_RGBA` | R | G | B | A |  |

The red, green, blue, alpha, luminance, and/or intensity components of the resulting pixels are stored in floating-point rather than integer format.

Pixel ordering is such that lower x screen coordinates correspond to lower i filter image coordinates, and lower y screen coordinates correspond to lower j filter image coordinates.

Note that after a convolution is performed, the resulting color components are also scaled by their corresponding `GL_POST_CONVOLUTION_c_SCALE` parameters and biased by their corresponding `GL_POST_CONVOLUTION_c_BIAS` parameters (where c takes on the values RED, GREEN, BLUE, and ALPHA). These parameters are set by glPixelTransfer.

## Notes

`glCopyConvolutionFilter2D` is present only if `ARB_imaging` is returned when glGetString is called with an argument of `GL_EXTENSIONS`.

## Errors

`GL_INVALID_ENUM` is generated if `target` is not `GL_CONVOLUTION_2D`.

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

`GL_INVALID_VALUE` is generated if `width` is less than zero or greater than the maximum supported value. This value may be queried with glGetConvolutionParameter using target `GL_CONVOLUTION_2D` and name `GL_MAX_CONVOLUTION_WIDTH`.

`GL_INVALID_VALUE` is generated if `height` is less than zero or greater than the maximum supported value. This value may be queried with glGetConvolutionParameter using target `GL_CONVOLUTION_2D` and name `GL_MAX_CONVOLUTION_HEIGHT`.

`GL_INVALID_OPERATION` is generated if `glCopyConvolutionFilter2D` is executed between the execution of glBegin and the corresponding execution of glEnd.

## Associated Gets

glGetConvolutionParameter, glGetConvolutionFilter

## See Also

glConvolutionFilter2D, glConvolutionParameter, glPixelTransfer

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.