GLPIXELSTORE()GLPIXELSTORE()NAME
glPixelStoref, glPixelStorei - set pixel storage modes
C SPECIFICATION
void glPixelStoref( GLenum pname,
GLfloat param )
void glPixelStorei( GLenum pname,
GLint param )
delim $$
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.
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_ALIGN‐
MENT.
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 twelve 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 $b sub
0$, $b sub 1$, $b sub 2$, $b sub 3$, it is stored in memory
as $b sub 3$, $b sub 2$, $b sub 1$, $b sub 0$ 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
$k ~=~ left { lpile { n l above {a over s left ceiling { s n
l } over a right ceiling}} ~~ lpile {s >= a above s < a }$
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 $width$
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
$k ~=~ 8 a left ceiling { n l } over { 8 a } right ceiling$
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 $i n$ 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 $j k$ components or indices, where $k$ is the num‐
ber 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 six of the twelve storage parameters affect how pixel data is
read from client memory. These values are significant for glDrawPix‐
els, glTexImage1D, glTexImage2D, glTexSubImage1D, glTexSubImage2D,
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 $b sub 0$, $b sub
1$, $b sub 2$, $b sub 3$, it is taken from memory as $b sub 3$,
$b sub 2$, $b sub 1$, $b sub 0$ if GL_UNPACK_SWAP_BYTES is true.
GL_UNPACK_SWAP_BYTES has no effect on the memory order of compo‐
nents within a pixel, only on the order of bytes within compo‐
nents 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
$k ~=~ left { lpile { n l above {a over s left ceiling { s n l
} over a right ceiling}} ~~ lpile {s >= a above s < a }$
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
$k ~=~ 8 a left ceiling { n l } over { 8 a } right ceiling$
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
equivalent to incrementing the pointer by $i n$ 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 $j k$ components or indices, where
$k$ is the number of components or indices per row, as just com‐
puted 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 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 │
└──────────────────────┴─────────┴───────────────┴───────────────┘
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.
Pixel storage modes are client state and must be pushed and restored
using
glPushClientAttrib and glPopClientAttrib.
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
SEE ALSO
glBitmap, glDrawPixels, glPixelMap, glPixelTransfer, glPixelZoom,
glPolygonStipple, glPushClientAttrib, glReadPixels, glTexImage1D,
glTexImage2D, glTexSubImage1D, glTexSubImage2D
GLPIXELSTORE()
