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GLPIXELSTORE(3gl)					     GLPIXELSTORE(3gl)

NAME
       glPixelStoref, glPixelStorei - set pixel storage modes

C SPECIFICATION
       void glPixelStoref( GLenum pname,
			   GLfloat param )
       void glPixelStorei( GLenum pname,
			   GLint param )

PARAMETERS
       pname  Specifies	 the  symbolic	name  of the parameter to be set.  Six
	      values  affect  the  packing  of	 pixel	 data	into   memory:
	      GL_PACK_SWAP_BYTES,    GL_PACK_LSB_FIRST,	   GL_PACK_ROW_LENGTH,
	      GL_PACK_SKIP_PIXELS, GL_PACK_SKIP_ROWS,  and  GL_PACK_ALIGNMENT.
	      Seven  more  affect  the	unpacking  of  pixel data from memory:
	      GL_UNPACK_SWAP_BYTES, GL_UNPACK_LSB_FIRST, GL_UNPACK_ROW_LENGTH,
	      GL_UNPACK_SKIP_PIXELS,  GL_UNPACK_SKIP_ROWS, GL_UNPACK_ALIGNMENT
	      and GL_UNPACK_CONSTANT_DATA_SUNX.

       param  Specifies the value that pname is set to.

DESCRIPTION
       glPixelStore sets pixel storage modes that affect the operation of sub‐
       sequent glDrawPixels and glReadPixels as well as the unpacking of poly‐
       gon stipple patterns (see glPolygonStipple),  bitmaps  (see  glBitmap),
       and  texture patterns (see glTexImage1D, glTexImage2D, glTexSubImage1D,
       and glTexSubImage2D).

       pname is a symbolic constant indicating the parameter to	 be  set,  and
       param  is the new value.	 Six of the thirteen storage parameters affect
       how pixel data is returned to client memory, and are therefore signifi‐
       cant only for glReadPixels commands.  They are as follows:

       GL_PACK_SWAP_BYTES
		 If  true, byte ordering for multibyte color components, depth
		 components, color indices, or stencil	indices	 is  reversed.
		 That  is,  if a four-byte component consists of bytes b0, b1,
		 b2, b3, it  is	 stored	 in  memory  as	 b3,  b2,  b1,	b0  if
		 GL_PACK_SWAP_BYTES is true.  GL_PACK_SWAP_BYTES has no effect
		 on the memory order of components within a pixel, only on the
		 order	of  bytes  within components or indices.  For example,
		 the three components of a  GL_RGB  format  pixel  are	always
		 stored	 with red first, green second, and blue third, regard‐
		 less of the value of GL_PACK_SWAP_BYTES.

       GL_PACK_LSB_FIRST
		 If true, bits are ordered within a byte from  least  signifi‐
		 cant  to  most	 significant; otherwise, the first bit in each
		 byte is the most significant one.  This parameter is signifi‐
		 cant for bitmap data only.

       GL_PACK_ROW_LENGTH
		 If  greater  than 0, GL_PACK_ROW_LENGTH defines the number of
		 pixels in a row.  If the first pixel of a row	is  placed  at
		 location p in memory, then the location of the first pixel of
		 the next row is obtained by skipping
					     nl	   s≥a

					  k=⎩

					     _⎡___s<a

		 components or indices, where n is the number of components or
		 indices  in  a	 pixel,	 l  is	the  number of pixels in a row
		 (GL_PACK_ROW_LENGTH if it is greater than 0, the width	 argu‐
		 ment  to  the	pixel  routine	otherwise),  a is the value of
		 GL_PACK_ALIGNMENT, and s is the size, in bytes, of  a	single
		 component  (if	 a<s,  then  it is as if a=s).	In the case of
		 1-bit values, the location of the next	 row  is  obtained  by
		 skipping

					   k=8a⎡__⎤

		 components or indices.

		 The word component in this description refers to the nonindex
		 values red, green, blue, alpha, and  depth.   Storage	format
		 GL_RGB,  for  example,	 has three components per pixel: first
		 red, then green, and finally blue.

       GL_PACK_SKIP_PIXELS and GL_PACK_SKIP_ROWS
		 These values are provided as a convenience to the programmer;
		 they  provide no functionality that cannot be duplicated sim‐
		 ply by incrementing the pointer passed to glReadPixels.  Set‐
		 ting  GL_PACK_SKIP_PIXELS  to i is equivalent to incrementing
		 the pointer by in components or indices, where n is the  num‐
		 ber   of  components  or  indices  in	each  pixel.   Setting
		 GL_PACK_SKIP_ROWS to j	 is  equivalent	 to  incrementing  the
		 pointer by jk components or indices, where k is the number of
		 components or indices	per  row,  as  just  computed  in  the
		 GL_PACK_ROW_LENGTH section.

       GL_PACK_ALIGNMENT
		 Specifies  the	 alignment  requirements for the start of each
		 pixel row in memory.  The allowable values are 1 (byte-align‐
		 ment),	 2  (rows  aligned  to	even-numbered bytes), 4 (word-
		 alignment), and 8 (rows start on double-word boundaries).

       The other seven of the thirteen storage	parameters  affect  how	 pixel
       data  is	 read  from  client  memory.  These values are significant for
       glDrawPixels, glTexImage1D, glTexImage2D, glTexSubImage1D,  glTexSubIm‐
       age2D, glBitmap, and glPolygonStipple.  They are as follows:

       GL_UNPACK_SWAP_BYTES
	      If  true,	 byte  ordering	 for multibyte color components, depth
	      components, color indices, or stencil indices is reversed.  That
	      is,  if  a four-byte component consists of bytes b0, b1, b2, b3,
	      it   is	taken	from   memory	as   b3,   b2,	 b1,   b0   if
	      GL_UNPACK_SWAP_BYTES   is	 true.	 GL_UNPACK_SWAP_BYTES  has  no
	      effect on the memory order of components within a pixel, only on
	      the  order  of bytes within components or indices.  For example,
	      the three components of a GL_RGB format pixel are always	stored
	      with  red first, green second, and blue third, regardless of the
	      value of GL_UNPACK_SWAP_BYTES.

       GL_UNPACK_LSB_FIRST
	      If true, bits are ordered within a byte from  least  significant
	      to  most	significant;  otherwise, the first bit in each byte is
	      the most significant one.	 This  is  relevant  only  for	bitmap
	      data.

       GL_UNPACK_ROW_LENGTH
	      If  greater  than	 0, GL_UNPACK_ROW_LENGTH defines the number of
	      pixels in a row.	If the first pixel of a row is placed at loca‐
	      tion  p  in  memory, then the location of the first pixel of the
	      next row is obtained by skipping

					k=⎩

					   _⎡___s<a

	      components or indices, where n is the number  of	components  or
	      indices  in  a  pixel,  l	 is  the  number  of  pixels  in a row
	      (GL_UNPACK_ROW_LENGTH if it is greater than 0, the  width	 argu‐
	      ment  to	the  pixel  routine  otherwise),  a  is	 the  value of
	      GL_UNPACK_ALIGNMENT, and s is the size, in bytes,	 of  a	single
	      component	 (if a<s, then it is as if a=s).  In the case of 1-bit
	      values, the location of the next row is obtained by skipping

					  k=8a⎡__⎤

	      components or indices.

	      The word component in this description refers  to	 the  nonindex
	      values  red,  green,  blue,  alpha,  and	depth.	Storage format
	      GL_RGB, for example, has three components per pixel: first  red,
	      then green, and finally blue.

       GL_UNPACK_SKIP_PIXELS and GL_UNPACK_SKIP_ROWS
	      These  values  are  provided as a convenience to the programmer;
	      they provide no  functionality  that  cannot  be	duplicated  by
	      incrementing  the	 pointer passed to glDrawPixels, glTexImage1D,
	      glTexImage2D,  glTexSubImage1D,  glTexSubImage2D,	 glBitmap,  or
	      glPolygonStipple.	 Setting GL_UNPACK_SKIP_PIXELS to i is equiva‐
	      lent to incrementing the pointer by in  components  or  indices,
	      where  n	is  the number of components or indices in each pixel.
	      Setting GL_UNPACK_SKIP_ROWS to j is equivalent  to  incrementing
	      the  pointer  by jk components or indices, where k is the number
	      of components or indices	per  row,  as  just  computed  in  the
	      GL_UNPACK_ROW_LENGTH section.

       GL_UNPACK_ALIGNMENT
	      Specifies the alignment requirements for the start of each pixel
	      row in memory.  The allowable values are 1  (byte-alignment),  2
	      (rows aligned to even-numbered bytes), 4 (word-alignment), and 8
	      (rows start on double-word boundaries).

       GL_UNPACK_CONSTANT_DATA_SUNX
	      If true, application specified data pointer may be used as-is by
	      the  library  without  making  a	second	copy  internally. This
	      implies that the application cannot  change  this	 data  pointer
	      once  it has been sent down to the library. Whether the applica‐
	      tion specified constant data is used as-is internally is	imple‐
	      mentation	 dependent.  An implementation may still make a second
	      copy even when this value is set to GL_TRUE.

       The following table gives the type, initial value, and range  of	 valid
       values for each storage parameter that can be set with glPixelStore.

       ┌─────────────────────────────┬─────────┬───────────────┬───────────────┐
       │	   pname	     │	type   │ initial value │  valid range  │
       ├─────────────────────────────┼─────────┼───────────────┼───────────────┤
       │     GL_PACK_SWAP_BYTES	     │ boolean │     false     │ true or false │
       │     GL_PACK_LSB_FIRST	     │ boolean │     false     │ true or false │
       │     GL_PACK_ROW_LENGTH	     │ integer │       0       │     [0,∞)     │
       │     GL_PACK_SKIP_ROWS	     │ integer │       0       │     [0,∞)     │
       │    GL_PACK_SKIP_PIXELS	     │ integer │       0       │     [0,∞)     │
       │     GL_PACK_ALIGNMENT	     │ integer │       4       │ 1, 2, 4, or 8 │
       ├─────────────────────────────┼─────────┼───────────────┼───────────────┤
       │    GL_UNPACK_SWAP_BYTES     │ boolean │     false     │ true or false │
       │    GL_UNPACK_LSB_FIRST	     │ boolean │     false     │ true or false │
       │    GL_UNPACK_ROW_LENGTH     │ integer │       0       │     [0,∞)     │
       │    GL_UNPACK_SKIP_ROWS	     │ integer │       0       │     [0,∞)     │
       │   GL_UNPACK_SKIP_PIXELS     │ integer │       0       │     [0,∞)     │
       │    GL_UNPACK_ALIGNMENT	     │ integer │       4       │ 1, 2, 4, or 8 │
       │GL_UNPACK_CONSTANT_DATA_SUNX │ boolean │     false     │ true or false │
       └─────────────────────────────┴─────────┴───────────────┴───────────────┘
       glPixelStoref  can  be  used  to set any pixel store parameter.	If the
       parameter type is boolean, then if param is 0, the parameter is	false;
       otherwise  it  is  set  to true.	 If pname is a integer type parameter,
       param is rounded to the nearest integer.

       Likewise, glPixelStorei can also be used to set any of the pixel	 store
       parameters.  Boolean parameters are set to false if param is 0 and true
       otherwise.

NOTES
       The pixel storage modes	in  effect  when  glDrawPixels,	 glReadPixels,
       glTexImage1D, glTexImage2D, glTexSubImage1D, glTexSubImage2D, glBitmap,
       or glPolygonStipple is placed in a display list control the interpreta‐
       tion  of memory data.  The pixel storage modes in effect when a display
       list is executed are not significant. At this time,  if	GL_UNPACK_CON‐
       STANT_DATA_SUNX	is  True  and all other pixel store modes are default,
       then the application specified data pointer is used internally. In this
       case,  this constant data pointer should remain unchanged for the life‐
       time of the display list. Changing the data pointer after it  has  been
       defined	as  "constant"	during display list compilation will result in
       unpredictable results.

       Pixel storage modes are client state and must be	 pushed	 and  restored
       using glPushClientAttrib and glPopClientAttrib.

       The  extension  GL_SUNX_CONSTANT_DATA should be defined in order to use
       GL_UNPACK_CONSTANT_DATA_SUNX.

       If GL_UNPACK_CONSTANT_DATA_SUNX is True, but some of  the  other	 pixel
       store  values  for  unpacking  the  pixels  is  non-default,  then  the
       GL_UNPACK_CONSTANT_DATA_SUNX value is ignored.

ERRORS
       GL_INVALID_ENUM is generated if pname is not an accepted value.

       GL_INVALID_VALUE is generated if a negative row length, pixel skip,  or
       row skip value is specified, or if alignment is specified as other than
       1, 2, 4, or 8.

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

ASSOCIATED GETS
       glGet with argument GL_PACK_SWAP_BYTES
       glGet with argument GL_PACK_LSB_FIRST
       glGet with argument GL_PACK_ROW_LENGTH
       glGet with argument GL_PACK_SKIP_ROWS
       glGet with argument GL_PACK_SKIP_PIXELS
       glGet with argument GL_PACK_ALIGNMENT
       glGet with argument GL_UNPACK_SWAP_BYTES
       glGet with argument GL_UNPACK_LSB_FIRST
       glGet with argument GL_UNPACK_ROW_LENGTH
       glGet with argument GL_UNPACK_SKIP_ROWS
       glGet with argument GL_UNPACK_SKIP_PIXELS
       glGet with argument GL_UNPACK_ALIGNMENT
       glGet with argument GL_UNPACK_CONSTANT_DATA_SUNX

SEE ALSO
       glBitmap,   glDrawPixels,   glPixelMap,	glPixelTransfer,  glPixelZoom,
       glPolygonStipple,   glPushClientAttrib,	 glReadPixels,	 glTexImage1D,
       glTexImage2D, glTexSubImage1D, glTexSubImage2D

				   15 Mar 97		     GLPIXELSTORE(3gl)
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