Using the Histogram Extension

1. Call glHistogramExt() to define the histogram, for example:

   glHistogramEXT(GL_HISTOGRAM_EXT,

                  256               /* width (number of bins) */,
                  GL_LUMINANCE      /* internalformat */,
                  GL_TRUE           /* sink */);

• width, which must be a power of 2, specifies the number of entries in the histogram.

• internalformat specifies the format of each table entry.

• sink specifies whether pixel groups are consumed by the histogram operation (GL_TRUE) or passed further down the image pipeline (GL_FALSE).

1. Enable histogramming by calling

   glEnable(GL_HISTOGRAM_EXT)

2. Perform the pixel operations for which you want to collect information (drawing, reading, or copying pixels).

   For each component represented in the histogram internal format, let the corresponding component of the incoming pixel (luminance corresponds to red) be of value c (after clamping to [0, 1). The corresponding component of bin number floor((width-1)*c) is incremented by 1.

3. Call glGetHistogramEXT() to query the current contents of the histogram:

   void glGetHistogramEXT( GLenum target, GLboolean reset, GLenum format,

   GLenum type, GLvoid *values )

   • target must be GL_HISTOGRAM_EXT.

   • reset is either GL_TRUE or GL_FALSE. If GL_TRUE, each component counter that is actually returned is reset to zero. Counters that are not returned are not modified.

   • format must be one of GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGBA, GL_RGB, GL_ABGR_EXT, GL_LUMINANCE, or GL_LUMINANCE_ALPHA.

   • type must be GL_UNSIGNED_BYTE, GL_BYTE, GL_UNSIGNED_SHORT, GL_SHORT, GL_UNSIGNED_INT, GL_INT, or GL_FLOAT.

   • values is used to return a 1D image with the same width as the histogram. No pixel transfer operations are performed on this image, but pixel storage modes that apply for glReadPixels() are performed. Color components that are requested in the specified format, but are not included in the internal format of the histogram, are returned as zero. The assignments of internal color components to the components requested by format are as follows:

     internal component	resulting component
     red	red
     green	green
     blue	blue
     alpha	alpha
     luminance	red/luminance