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GLGETHISTOGRAMEXT(3GL)					GLGETHISTOGRAMEXT(3GL)

NAME
       glGetHistogramEXT,  glGetHistogramParameterivEXT, glGetHistogramParame‐
       terfvEXT.  - gets histogram image and histogram related parameters

C SPECIFICATION
       void glGetHistogramEXT ( GLenum target,
	      GLboolean reset,
	      GLenum format,
	      GLenum type,
	      GLvoid *values)

PARAMETERS
       target	The target histogram to be queried for	the  histogram	image.
		Must be GL_HISTOGRAM_EXT.

       reset	Specifies  whether  or not to reset the histogram image compo‐
		nents that have been queried. Must be GL_TRUE or GL_FALSE.

       format	Format the caller wants the histogram returned in. Must be one
		of  GL_RED,  GL_GREEN,	GL_BLUE,  GL_ALPHA,  GL_RGB,  GL_RGBA,
		GL_ABGR_EXT, GL_LUMINANCE, or GL_LUMINANCE_ALPHA.

       type	Data type the caller wants the histogram returned in. Must  be
		one of GL_BYTE, GL_UNSIGNED_BYTE, GL_SHORT, GL_UNSIGNED_SHORT,
		GL_INT, GL_UNSIGNED_INT, or GL_FLOAT.

       value
		Pointer to area of memory large enough to contain the returned
		histogram  image.  (Should  have  width*num_format_components*
		sizeof(type) bytes. The value of width is the  same  that  was
		used when defining the current histogram with a call to glHis‐
		togramEXT.)

C SPECIFICATION
       void glGetHistogramParameterivEXT ( GLenum target,
	      GLenum pname,
	      GLint* params)
       void glGetHistogramParameterfvEXT ( GLenum target,
	      GLenum pname,
	      GLfloat* params)

PARAMETERS
       target	       The target histogram to be  queried.  Must  be  GL_HIS‐
		       TOGRAM_EXT or GL_PROXY_HISTOGRAM_EXT.

       pname	       Parameter  name. Must be one of GL_HISTOGRAM_WIDTH_EXT,
		       GL_HISTOGRAM_FORMAT_EXT,	    GL_HISTOGRAM_RED_SIZE_EXT,
		       GL_HISTOGRAM_GREEN_SIZE_EXT,		       GL_HIS‐
		       TOGRAM_BLUE_SIZE_EXT,	  GL_HISTOGRAM_ALPHA_SIZE_EXT,
		       GL_HISTOGRAM_LUMINANCE_SIZE_EXT,	      or       GL_HIS‐
		       TOGRAM_SINK_EXT.

       params	       Pointer to a single integer or  floating	 point	memory
		       location to contain the retrieved value.

DESCRIPTION
       glGetHistogramEXT, glGetHistogramParameterivEXT and glGetHistogramPara‐
       meterivEXT are part of the EXT_histogram extension, which defines  his‐
       togram  and minmax operations at a fixed location in the pixel transfer
       pipeline after convolution and post convolution colortable.

       The current contents of the histogram tables are queried using  glGetH‐
       istogramEXT. <target> must be GL_HISTOGRAM_EXT. <format> must be one of
       GL_RED, GL_GREEN,  GL_BLUE,  GL_ALPHA,  GL_RGB,	GL_RGBA,  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. A 1-dimensional image with the same width as the histogram is
       returned to <values>. No pixel transfer	operations  are	 performed  on
       this  image,  but pixel storage modes that are applicable to ReadPixels
       are performed. Color components that are	 requested  in	the  specified
       <format>, but which are not included in the internal format of the his‐
       togram, are returned as zero. The assignments of internal color	compo‐
       nents to the components requested by <format> are:

	    Internal Component	 Resulting Component
	   __________________________________________
		  red			red
		  green			green
		  blue			blue
		  alpha			alpha
		  luminance		red
	   __________________________________________

       <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.

       The  query functions glGetHistogramParameterivEXT and glGetHistogramPa‐
       rameterfvEXT are provided to retrieve that state of the current defini‐
       tion  of	 the current histogram, which were set with the call to glHis‐
       togramEXT.  The <target>	 must  be  GL_HISTOGRAM_EXT  or	 GL_PROXY_HIS‐
       TOGRAM_EXT.  The	 <pname>  is  one  of GL_HISTOGRAM_FORMAT_EXT, GL_HIS‐
       TOGRAM_WIDTH_EXT,	  GL_HISTOGRAM_RED_SIZE_EXT,	       GL_HIS‐
       TOGRAM_GREEN_SIZE_EXT,	     GL_HISTOGRAM_BLUE_SIZE_EXT,       GL_HIS‐
       TOGRAM_ALPHA_SIZE_EXT,  GL_HISTOGRAM_LUMINANCE_SIZE_EXT,	  or   GL_HIS‐
       TOGRAM_SINK_EXT.	 The  value  of the specified parameter is returned in
       <params>.

       Alternate sets of partial histogram state are  defined  for  the	 proxy
       histogram GL_PROXY_HISTOGRAM_EXT. Specifically, GL_HISTOGRAM_WIDTH_EXT,
       GL_HISTOGRAM_FORMAT,	    GL_HISTOGRAM_RED_SIZE_EXT,	       GL_HIS‐
       TOGRAM_GREEN_SIZE_EXT,	     GL_HISTOGRAM_BLUE_SIZE_EXT,       GL_HIS‐
       TOGRAM_ALPHA_SIZE_EXT, and  GL_HISTOGRAM_LUMINANCE_SIZE_EXT  are	 main‐
       tained  for  the	 proxy	histogram.  When glHistogramEXT is called with
       <target> set to GL_PROXY_HISTOGRAM_EXT, these proxy  state  values  are
       always  recomputed  and	updated, even if the histogram is too large to
       actually be used. If the histogram is too large,	 all  of  these	 state
       variables  are  set  to zero. If the histogram could be accommodated by
       Histogram called with <target> GL_HISTOGRAM_EXT, these values  are  set
       as  though GL_HISTOGRAM_EXT were being defined. All of these state val‐
       ues can be queried with glGetHistogramParameterivEXT or glGetHistogram‐
       ParameterfvEXT  with  <target>  set  to GL_PROXY_HISTOGRAM_EXT. Calling
       glHistogramEXT with <target> GL_PROXY_HISTOGRAM_EXT has	no  effect  on
       the actual histogram.

       There  is  no  image  associated with GL_PROXY_HISTOGRAM_EXT. Therefore
       GL_PROXY_HISTOGRAM_EXT cannot be used as a  histogram,  and  its	 image
       must   never   be   queried   using   glGetHistogramEXT.	   (The	 error
       GL_INVALID_ENUM results if this is attempted.)

NOTES
       Doing a query using glGetHistogramParameterivEXT or glGetHistogramPara‐
       meterfvEXT with the <target> set to  GL_PROXY_HISTOGRAM_EXT and <pname>
       set to GL_HISTOGRAM_SINK_EXT will generate  an  error  GL_INVALID_ENUM.
       GL_HISTOGRAM_SINK_EXT only applies to the <target> GL_HISTOGRAM_EXT.

       glGetHistogramEXT,  glGetHistogramParameterivEXT, glGetHistogramParame‐
       terfvEXT are not included in display lists. They are  executed  immedi‐
       ately when called.

ERRORS
       GL_INVALID_ENUM is generated if glGetHistogramEXT parameter <format> is
       not GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_RGBA,  GL_ABGR_EXT,
       GL_LUMINANCE, or GL_LUMINANCE_ALPHA.

       GL_INVALID_ENUM	is  generated if glGetHistogramEXT parameter <type> is
       not    GL_UNSIGNED_BYTE,	   GL_BYTE,    GL_UNSIGNED_SHORT,    GL_SHORT,
       GL_UNSIGNED_INT, GL_INT, or GL_FLOAT.

       GL_INVALID_ENUM is generated if glGetHistogramParameterivEXT or glGetH‐
       istogramParameterfvEXT parameter <target> is  not  GL_HISTOGRAM_EXT  or
       GL_PROXY_HISTOGRAM_EXT.

       GL_INVALID_ENUM is generated if glGetHistogramParameterivEXT or glGetH‐
       istogramParameterfvEXT  parameter  <pname>  is  not   GL_HISTOGRAM_FOR‐
       MAT_EXT,	  GL_HISTOGRAM_WIDTH_EXT,  GL_HISTOGRAM_RED_SIZE_EXT,  GL_HIS‐
       TOGRAM_GREEN_SIZE_EXT,	    GL_HISTOGRAM_BLUE_SIZE_EXT,	       GL_HIS‐
       TOGRAM_ALPHA_SIZE_EXT,	GL_HISTOGRAM_LUMINANCE_SIZE_EXT,   or  GL_HIS‐
       TOGRAM_SINK_EXT. GL_INVALID_ENUM is also generated if glGetHistogramPa‐
       rameterivEXT  or glGetHistogramParameterfvEXT parameter <target> speci‐
       fies the proxy histogram and <pname> specifies a piece of  state	 which
       is not maintained for the proxy histogram.

       GL_INVALID_OPERATION  is	 generated  if	glGetHistogramEXT,   glGetHis‐
       togramParameterivEXT,  or   glGetHistogramParameterfvEXT	  are	called
       between execution of glBegin and the corresponding execution of glEnd.

ASSOCIATED CALLS
       glHistogramEXT and glResetHistogramEXT.

SEE ALSO
       glMinmaxEXT,   glResetMinmaxEXT,	  glGetMinmaxEXT,   glGetMinmaxParame‐
       terivEXT,   glGetMinmaxParameterfvEXT,	glDrawPixels,	 glCopyPixels,
       glReadPixels,	glTexImage2D,	glTexImage3DEXT,   glTexSubImage2DEXT,
       glCopyTexImage2DEXT,  glCopyTexSubImage2DEXT,  glGetTexImage,  glPixel‐
       Tranferf,  glPixelTransferi, glEnable, glDisable, glIsEnabled, glPixel‐
       Store.

				   1 Dec 97		GLGETHISTOGRAMEXT(3GL)
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