glPixelStoref, glPixelStorei - set pixel storage modes
void glPixelStoref( GLenum pname, GLfloat param ) void glPixelStorei( GLenum pname, GLint param )
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. Six 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, and GL_UNPACK_ALIGNMENT. param Specifies the value that pname is set to.
glPixelStore sets pixel storage modes that affect the operation of
subsequent glDrawPixels and glReadPixels as well as the unpacking of
polygon stipple patterns (see glPolygonStipple), bitmaps (see glBitmap),
and texture patterns (see glTexImage1D and glTexImage2D).
pname is a symbolic constant indicating the parameter to be set, and param
is the new value. Six of the twelve storage parameters affect how pixel
data is returned to client memory, and are therefore significant 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 is made up of bytes b , b , b , b ,
0 1 2 3
it is stored in memory as b , b , b , b if GL_PACK_SWAP_BYTES is
3 2 1 0
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, regardless of the value of
GL_PACK_SWAP_BYTES.
GL_PACK_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 parameter is significant for bitmap
data only.
GL_PACK_ROW_LENGTH
If greater than zero, 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
k = nl if s >= a
snl
k = a/s·ceil( --- ) if s < a
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 zero, the width
argument 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
nl
k = 8a·ceil( -- )
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 simply by
incrementing the pointer passed to glReadPixels. Setting
GL_PACK_SKIP_PIXELS to i is equivalent to incrementing the
pointer by in components or indices, where n is the number 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 computed above 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-alignment), 2
(rows aligned to even-numbered bytes), 4 (word alignment), and 8
(rows start on double-word boundaries).
The other six of the twelve storage parameters affect how pixel data is
read from client memory. These values are significant for glDrawPixels,
glTexImage1D, glTexImage2D, 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 is made up of bytes b , b , b , b , it is
0 1 2 3
taken from memory as b , b , b , b if GL_UNPACK_SWAP_BYTES is true.
3 2 1 0
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 significant for bitmap data only.
GL_UNPACK_ROW_LENGTH
If greater than zero, GL_UNPACK_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
k = nl if s >= a
snl
k = a/s·ceil( --- ) if s < a
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 zero, the width argument 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
nl
k = 8a·ceil( -- )
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 simply by
incrementing the pointer passed to glDrawPixels, glTexImage1D,
glTexImage2D, glBitmap, or glPolygonStipple. Setting
GL_UNPACK_SKIP_PIXELS to i is equivalent 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 computed above 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).
The following table gives the type, initial value, and range of valid
values for each of the storage parameters that can be set with
glPixelStore.
-----------------------------------------------------------------
| parameter name | 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,oo) |
| GL_PACK_SKIP_ROWS | integer | 0 | [0,oo) |
| GL_PACK_SKIP_PIXELS | integer | 0 | [0,oo) |
| 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,oo) |
| GL_UNPACK_SKIP_ROWS | integer | 0 | [0,oo) |
|GL_UNPACK_SKIP_PIXELS | integer | 0 | [0,oo) |
| GL_UNPACK_ALIGNMENT | integer | 4 | 1, 2, 4, or 8 |
-----------------------------------------------------------------
glPixelStoref can be used to set any pixel store parameter. If the
parameter type is Boolean, then if param is 0.0, 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. param is converted to floating point before being assigned to
real-valued parameters.
The pixel storage modes in effect when glDrawPixels, glReadPixels, glTexImage1D, glTexImage2D, glBitmap, or glPolygonStipple is placed in a display list control the interpretation of memory data. The pixel storage modes in effect when a display list is executed are not significant.
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 called between a call to glBegin and the corresponding call to glEnd.
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
glBitmap, glDrawPixels, glPixelMap, glPixelTransfer, glPixelZoom, glPolygonStipple, glReadPixels, glTexImage1D, glTexImage2D
Introduction | Alphabetic | Specification
Last Edited: Mon, May 22, 1995