GLenum internalformat, GLint x, GLint y, GLsizei width, GLsizei height )
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 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 (red, green, blue, alpha, luminance, and intensity). The mapping is as follows:
|Internal Format Red Green Blue Alpha Luminance Intensity|
|GL_LUMINANCE_ALPHA A 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
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.
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.
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