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_ALIGNMENT.
param Specifies the value that pname is set to.
DESCRIPTION
glPixelStore sets pixel storage modes that affect the opera-
tion of subsequent glDrawPixels and glReadPixels as well as
the unpacking of polygon stipple patterns (see glPolygon-
Stipple), 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 com-
ponents, depth components, color indices, or sten-
cil indices is reversed. That is, if a four-byte
component is made up 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, 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 = 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 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
$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 simply by incrementing the
pointer passed to glReadPixels. Setting
GL_PACK_SKIP_PIXELS to $i$ is equivalent to incre-
menting 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
number of components or indices per row, as com-
puted 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 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 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 sig-
nificant to most significant; otherwise, the first bit
in each byte is the most significant one. This is sig-
nificant 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 skip-
ping
$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 com-
ponents 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
$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 com-
ponents 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 pro-
grammer; 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 $i n$ com-
ponents or indices, where $n$ is the number of com-
ponents or indices in each pixel. Setting
GL_UNPACK_SKIP_ROWS to $j$ is equivalent to increment-
ing the pointer by $j k$ 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.
center box tab(:) delim($$) ; ci | ci | ci | ci c | c | c |
c . 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,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 parame-
ters.
NOTES
The pixel storage modes in effect when glDrawPixels, glRead-
Pixels, glTexImage1D, glTexImage2D, glBitmap, or glPolygon-
Stipple 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.
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 called
between a call to glBegin and the corresponding call to
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, glPix-
elZoom, glPolygonStipple, glReadPixels, glTexImage1D,
glTexImage2D
Acknowledgement and Disclaimer