glDrawPixels.3gl




Name

  glDrawPixels - write a block of pixels to the	frame buffer


C Specification

  void glDrawPixels( GLsizei width,
		     GLsizei height,
		     GLenum format,
		     GLenum type,
		     const GLvoid *pixels )


Parameters


  width, height	Specify	the dimensions of the pixel rectangle that will	be
		written	into the frame buffer.

  format	Specifies the format of	the pixel data.	 Symbolic constants
		GL_COLOR_INDEX,	GL_STENCIL_INDEX, GL_DEPTH_COMPONENT,
		GL_RGBA, GL_RED, GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB,
		GL_LUMINANCE, and GL_LUMINANCE_ALPHA are accepted.

  type		Specifies the data type	for pixels.  Symbolic constants
		GL_UNSIGNED_BYTE, GL_BYTE, GL_BITMAP, GL_UNSIGNED_SHORT,
		GL_SHORT, GL_UNSIGNED_INT, GL_INT, and GL_FLOAT	are accepted.

  pixels	Specifies a pointer to the pixel data.


Description

  glDrawPixels reads pixel data	from memory and	writes it into the frame
  buffer relative to the current raster	position.  Use glRasterPos to set the
  current raster position, and use glGet with argument
  GL_CURRENT_RASTER_POSITION to	query the raster position.

  Several parameters define the	encoding of pixel data in memory and control
  the processing of the	pixel data before it is	placed in the frame buffer.
  These	parameters are set with	four commands: glPixelStore, glPixelTransfer,
  glPixelMap, and  glPixelZoom.	 This reference	page describes the effects on
  glDrawPixels of many,	but not	all, of	the parameters specified by these
  four commands.

  Data is read from pixels as a	sequence of signed or unsigned bytes, signed
  or unsigned shorts, signed or	unsigned integers, or single-precision
  floating-point values, depending on type.  Each of these bytes, shorts,
  integers, or floating-point values is	interpreted as one color or depth
  component, or	one index, depending on	format.	 Indices are always treated
  individually.	 Color components are treated as groups	of one,	two, three,
  or four values, again	based on format.  Both individual indices and groups
  of components	are referred to	as pixels.  If type is GL_BITMAP, the data
  must be unsigned bytes, and format must be either GL_COLOR_INDEX or
  GL_STENCIL_INDEX.  Each unsigned byte	is treated as eight 1-bit pixels,
  with bit ordering determined by GL_UNPACK_LSB_FIRST (see glPixelStore).

  widthxheight pixels are read from memory, starting at	location pixels.  By
  default, these pixels	are taken from adjacent	memory locations, except that
  after	all width pixels are read, the read pointer is advanced	to the next
  four-byte boundary.  The four-byte row alignment is specified	by
  glPixelStore with argument GL_UNPACK_ALIGNMENT, and it can be	set to one,
  two, four, or	eight bytes.  Other pixel store	parameters specify different
  read pointer advancements, both before the first pixel is read, and after
  all width pixels are read.  Refer to the glPixelStore	reference page for
  details on these options.

  The widthxheight pixels that are read	from memory are	each operated on in
  the same way,	based on the values of several parameters specified by
  glPixelTransfer and glPixelMap.  The details of these	operations, as well
  as the target	buffer into which the pixels are drawn,	are specific to	the
  format of the	pixels,	as specified by	format.	 format	can assume one of
  eleven symbolic values:

  GL_COLOR_INDEX
	    Each pixel is a single value, a color index.  It is	converted to
	    fixed-point	format,	with an	unspecified number of bits to the
	    right of the binary	point, regardless of the memory	data type.
	    Floating-point values convert to true fixed-point values.  Signed
	    and	unsigned integer data is converted with	all fraction bits set
	    to zero.  Bitmap data convert to either 0.0	or 1.0.

	    Each fixed-point index is then shifted left	by GL_INDEX_SHIFT
	    bits and added to GL_INDEX_OFFSET.	If GL_INDEX_SHIFT is
	    negative, the shift	is to the right.  In either case, zero bits
	    fill otherwise unspecified bit locations in	the result.

	    If the GL is in RGBA mode, the resulting index is converted	to an
	    RGBA pixel using the GL_PIXEL_MAP_I_TO_R, GL_PIXEL_MAP_I_TO_G,
	    GL_PIXEL_MAP_I_TO_B, and GL_PIXEL_MAP_I_TO_A tables.  If the GL
	    is in color	index mode, and	if GL_MAP_COLOR	is true, the index is
	    replaced with the value that it references in lookup table
	    GL_PIXEL_MAP_I_TO_I.  Whether the lookup replacement of the	index
	    is done or not, the	integer	part of	the index is then ANDed	with
             b
	    2 -1, where	b is the number	of bits	in a color index buffer.

	    The	resulting indices or RGBA colors are then converted to
	    fragments by attaching the current raster position z coordinate
	    and	texture	coordinates to each pixel, then	assigning x and	y
	    window coordinates to the nth fragment such	that

				 x  = x	 + n mod width
				  n    r

   			         y  = y  + floor (n/width)
				   n	r

	    where (x ,y	) is the current raster	position.  These pixel
                    r  r
	    fragments are then treated just like the fragments generated by
	    rasterizing	points,	lines, or polygons.  Texture mapping, fog,
	    and	all the	fragment operations are	applied	before the fragments
	    are	written	to the frame buffer.

  GL_STENCIL_INDEX
	    Each pixel is a single value, a stencil index.  It is converted
	    to fixed-point format, with	an unspecified number of bits to the
	    right of the binary	point, regardless of the memory	data type.
	    Floating-point values convert to true fixed-point values.  Signed
	    and	unsigned integer data is converted with	all fraction bits set
	    to zero.  Bitmap data convert to either 0.0	or 1.0.

	    Each fixed-point index is then shifted left	by GL_INDEX_SHIFT
	    bits, and added to GL_INDEX_OFFSET.	 If GL_INDEX_SHIFT is
	    negative, the shift	is to the right.  In either case, zero bits
	    fill otherwise unspecified bit locations in	the result.  If
	    GL_MAP_STENCIL is true, the	index is replaced with the value that
	    it references in lookup table GL_PIXEL_MAP_S_TO_S.	Whether	the
	    lookup replacement of the index is done or not, the	integer	part
                                             b
	    of the index is then ANDed with 2 -1, where	b is the number	of
	    bits in the	stencil	buffer.	 The resulting stencil indices are
	    then written to the	stencil	buffer such that the nth index is
	    written to location

			       x  = x  + n mod width
				n    r

			       y  = y  + floor (n/width)
				n    r

       where (x	,y ) is	the current raster position.  Only the pixel
               r  r
       ownership test, the scissor test, and the stencil writemask affect
       these writes.

  GL_DEPTH_COMPONENT
       Each pixel is a single-depth component.	Floating-point data is
       converted directly to an	internal floating-point	format with
       unspecified precision.  Signed integer data is mapped linearly to the
       internal	floating-point format such that	the most positive
       representable integer value maps	to 1.0,	and the	most negative
       representable value maps	to -1.0.  Unsigned integer data	is mapped
       similarly: the largest integer value maps to 1.0, and zero maps to
       0.0.  The resulting floating-point depth	value is then multiplied by
       GL_DEPTH_SCALE and added	to GL_DEPTH_BIAS.  The result is clamped to
       the range [0,1].

       The resulting depth components are then converted to fragments by
       attaching the current raster position color or color index and texture
       coordinates to each pixel, then assigning x and y window	coordinates
       to the nth fragment such	that

			       x  = x  + n mod width
				n    r

			       y  = y  + floor (n/width)
				n    r

       where (x	,y ) is	the current raster position.  These pixel fragments
               r  r
       are then treated	just like the fragments	generated by rasterizing
       points, lines, or polygons.  Texture mapping, fog, and all the
       fragment	operations are applied before the fragments are	written	to
       the frame buffer.

  GL_RGBA
       Each pixel is a four-component group: red first,	followed by green,
       followed	by blue, followed by alpha.  Floating-point values are
       converted directly to an	internal floating-point	format with
       unspecified precision.  Signed integer values are mapped	linearly to
       the internal floating-point format such that the	most positive
       representable integer value maps	to 1.0,	and the	most negative
       representable value maps	to -1.0.  Unsigned integer data	is mapped
       similarly: the largest integer value maps to 1.0, and zero maps to
       0.0.  The resulting floating-point color	values are then	multiplied by
       GL_c_SCALE and added to GL_c_BIAS, where	c is RED, GREEN, BLUE, and
       ALPHA for the respective	color components.  The results are clamped to
       the range [0,1].

       If GL_MAP_COLOR is true,	each color component is	scaled by the size of
       lookup table GL_PIXEL_MAP_c_TO_c, then replaced by the value that it
       references in that table.  c is R, G, B,	or A, respectively.

       The resulting RGBA colors are then converted to fragments by attaching
       the current raster position z coordinate	and texture coordinates	to
       each pixel, then	assigning x and	y window coordinates to	the nth
       fragment	such that

			       x  = x  + n mod width
				n    r

			       y  = y  + floor (n/width)
				n    r

       where (x	,y ) is	the current raster position.  These pixel fragments
               r  r
       are then treated	just like the fragments	generated by rasterizing
       points, lines, or polygons.  Texture mapping, fog, and all the
       fragment	operations are applied before the fragments are	written	to
       the frame buffer.

  GL_RED
       Each pixel is a single red component.  This component is	converted to
       the internal floating-point format in the same way as the red
       component of an RGBA pixel is, then it is converted to an RGBA pixel
       with green and blue set to 0.0, and alpha set to	1.0.  After this
       conversion, the pixel is	treated	just as	if it had been read as an
       RGBA pixel.

  GL_GREEN
       Each pixel is a single green component.	This component is converted
       to the internal floating-point format in	the same way as	the green
       component of an RGBA pixel is, then it is converted to an RGBA pixel
       with red	and blue set to	0.0, and alpha set to 1.0.  After this
       conversion, the pixel is	treated	just as	if it had been read as an
       RGBA pixel.

  GL_BLUE
       Each pixel is a single blue component.  This component is converted to
       the internal floating-point format in the same way as the blue
       component of an RGBA pixel is, then it is converted to an RGBA pixel
       with red	and green set to 0.0, and alpha	set to 1.0.  After this
       conversion, the pixel is	treated	just as	if it had been read as an
       RGBA pixel.

  GL_ALPHA
       Each pixel is a single alpha component.	This component is converted
       to the internal floating-point format in	the same way as	the alpha
       component of an RGBA pixel is, then it is converted to an RGBA pixel
       with red, green,	and blue set to	0.0.  After this conversion, the
       pixel is	treated	just as	if it had been read as an RGBA pixel.

  GL_RGB
       Each pixel is a three-component group: red first, followed by green,
       followed	by blue.  Each component is converted to the internal
       floating-point format in	the same way as	the red, green,	and blue
       components of an	RGBA pixel are.	 The color triple is converted to an
       RGBA pixel with alpha set to 1.0.  After	this conversion, the pixel is
       treated just as if it had been read as an RGBA pixel.

  GL_LUMINANCE
       Each pixel is a single luminance	component.  This component is
       converted to the	internal floating-point	format in the same way as the
       red component of	an RGBA	pixel is, then it is converted to an RGBA
       pixel with red, green, and blue set to the converted luminance value,
       and alpha set to	1.0.  After this conversion, the pixel is treated
       just as if it had been read as an RGBA pixel.

  GL_LUMINANCE_ALPHA
       Each pixel is a two-component group: luminance first, followed by
       alpha.  The two components are converted	to the internal	floating-
       point format in the same	way as the red component of an RGBA pixel is,
       then they are converted to an RGBA pixel	with red, green, and blue set
       to the converted	luminance value, and alpha set to the converted	alpha
       value.  After this conversion, the pixel	is treated just	as if it had
       been read as an RGBA pixel.


  The following	table summarizes the meaning of	the valid constants for	the
  type parameter:

	 -------------------------------------------------------------
	 |      type        |          corresponding type            |
	 -------------------------------------------------------------
	 |GL_UNSIGNED_BYTE  |	      unsigned 8-bit integer	     |
	 |     GL_BYTE	    |	       signed 8-bit integer	     |
	 |    GL_BITMAP	    | single bits in unsigned 8-bit integers |
	 |GL_UNSIGNED_SHORT |	     unsigned 16-bit integer	     |
	 |    GL_SHORT	    |	      signed 16-bit integer	     |
	 | GL_UNSIGNED_INT  |	     unsigned 32-bit integer	     |
	 |     GL_INT	    |		  32-bit integer	     |
	 |    GL_FLOAT	    |	 single-precision floating-point     |
	 -------------------------------------------------------------

  The rasterization described thus far assumes pixel zoom factors of 1.0.  If
  glPixelZoom is used to change	the x and y pixel zoom factors,	pixels are
  converted to fragments as follows.  If (x , y ) is the current raster
                                           r   r
  position, and	a given	pixel is in the	nth column and mth row of the pixel
  rectangle, then fragments are	generated for pixels whose centers are in the
  rectangle with corners at

			      (x + zoom  n, y + zoom  m)
                                r      x     r      y

    		          (x + zoom  (n+1), y + zoom  (m+1))
                            r      x         r      y


  where	zoom  is the value of GL_ZOOM_X	and zoom  is the value of GL_ZOOM_Y.
            x                                   y

Errors

  GL_INVALID_VALUE is generated	if either width	or height is negative.

  GL_INVALID_ENUM is generated if format or type is not	one of the accepted
  values.

  GL_INVALID_OPERATION is generated if format is GL_RED, GL_GREEN, GL_BLUE,
  GL_ALPHA, GL_RGB, GL_RGBA, GL_LUMINANCE, or GL_LUMINANCE_ALPHA, and the GL
  is in	color index mode.

  GL_INVALID_ENUM is generated if type is GL_BITMAP and	format is not either
  GL_COLOR_INDEX or GL_STENCIL_INDEX.

  GL_INVALID_OPERATION is generated if format is GL_STENCIL_INDEX and there
  is no	stencil	buffer.

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

Associated Gets

  glGet	with argument GL_CURRENT_RASTER_POSITION
  glGet	with argument GL_CURRENT_RASTER_POSITION_VALID

See Also

  glAlphaFunc, glBlendFunc, glCopyPixels, glDepthFunc, glLogicOp, glPixelMap,
  glPixelStore,	glPixelTransfer, glPixelZoom, glRasterPos, glReadPixels,
  glScissor, glStencilFunc




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Last Edited: Mon, May 22, 1995

AFV