GLPIXELSTORE() MachTen Programmer’s Manual 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_ALIGNMENT.

param Specifies the value that pname is set to.

DESCRIPTION
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, 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 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
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, 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; other-
wise, 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 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 mem-
ory, 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 val-
ues, 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
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 incre-
menting the pointer by $j k$ 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 allow-
able 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,
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 components
within a pixel, only on the order of bytes within
components or indices. For example, the three com-
ponents 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 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_UNPACK_ROW_LENGTH if it is
greater than 0, 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 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 can-
not 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 incre-
menting the pointer by $j k$ 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).

The following table gives the type, initial value, and
range of valid values for each storage parameter 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 parame-
ter. 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 interpretation 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 align-
ment is specified as other than 1, 2, 4, or 8.

GL_INVALID_OPERATION is generated if glPixelStore is exe-
cuted between the execution of glBegin and the correspond-
ing 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, glTex-
Image1D, glTexImage2D, glTexSubImage1D, glTexSubImage2D

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