- set pixel storage modes
C SPECIFICATION
PARAMETERS
_param1 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_IMAGE_HEIGHT, GL_PACK_SKIP_PIXELS,
GL_PACK_SKIP_ROWS, GL_PACK_SKIP_IMAGES, 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_IMAGE_HEIGHT, GL_UNPACK_SKIP_PIXELS,
GL_UNPACK_SKIP_ROWS, GL_UNPACK_SKIP_IMAGES, and
GL_UNPACK_ALIGNMENT.
_param2 Specifies the value that _param1 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), texture pat-
terns (see glTexImage1D, glTexImage2D, glTexImage3D,
glTexSubImage1D, glTexSubImage2D, glTexSubImage3D). Addi-
tionally, if the GL_ARB_imaging extension is supported,
pixle storage modes affect convlution filters (see
glConvolutionFilter1D, glConvolutionFilter2D, and
glSeparableFilter2D, color table (see glColorTable, and
glColorSubTable, and unpacking histogram (See
glHistogram), and minmax (See glMinmax) data.
_param1 is a symbolic constant indicating the parameter to
be set, and _param2 is the new value. Six of the twelve
storage parameters affect how pixel data is returned to
client memory. 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 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
pixel are always stored with red first, green
second, and blue third, regardless of the value
next row is obtained by skipping
nl s>=a
k={
a<I>s_|s_n<I>a_l_|s<a
components or indices, where n is the number of
components or indices in a pixel, l is the num-
ber 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=8a|n_l<I>a_|
components or indices.
The word component in this description refers to
the nonindex values red, green, blue, alpha, and
depth. Storage GL_RGB, for example, has three
components per pixel: first red, then green, and
finally blue.
GL_PACK_IMAGE_HEIGHT
If greater than 0, GL_PACK_IMAGE_HEIGHT defines
the number of pixels in an image three-dimen-
sional texture volume. Where ``image'' is
defined by all pixels sharing the same third
dimension index. If the first pixel of a row is
placed at location p in memory, then the loca-
tion of the first pixel of the next row is
obtained by skipping
a<I>s_|s_n_l<I>a_h_|s<a
components or indices, where n is the number of
components or indices in a pixel, l is the num-
ber of pixels in a row (GL_PACK_ROW_LENGTH if it
is greater than 0, the width argument to
glTexImage3d otherwise), h is the number of rows
in a pixel image (GL_PACK_IMAGE_HEIGHT if it is
greater than 0, the height arguemnt to the
glTexImage3D 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).
The word component in this description refers to
the nonindex values red, green, blue, alpha, and
depth. Storage GL_RGB, for example, has three
components per pixel: first red, then green, and
finally blue.
GL_PACK_SKIP_PIXELS, GL_PACK_SKIP_ROWS, and
GL_PACK_SKIP_IMAGES
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 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 jm
components or indices, where m is the number of
components or indices per row, as just computed
in the GL_PACK_ROW_LENGTH section. Setting
GL_PACK_SKIP_IMAGES to k is equivalent to incre-
menting the pointer by kp, where p is the number
of components or indices per image, as computed
in the GL_PACK_IMAGE_HEIGHT 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-align-
ment), and 8 (rows start on double-word bound-
aries).
nents, depth components, color indices, or stencil
indices is reversed. That is, if a four-byte com-
ponent 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 com-
ponents of a GL_RGB 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
nl s>=a
k={
a<I>s_|s_n<I>a_l_|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,
GL_UNPACK_IMAGE_HEIGHT
If greater than 0, GL_UNPACK_IMAGE_HEIGHT defines
the number of pixels in an image of a three-dimen-
sional texture volume. Where ``image'' is defined
by all pixel sharing the same third dimension
index. 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
nlh s>=a
k={
a<I>s_|s_n_l<I>a_h_|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 glTexImage3D
otherwise), h is the number of rows in an image (-
GL_UNPACK_IMAGE_HEIGHT if it is greater than 0, the
height argument to glTexImage3D 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).
The word component in this description refers to
the nonindex values red, green, blue, alpha, and
depth. Storage 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,
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.
-----------------------------------------------------------------
_param1 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_IMAGE_HEIGHT integer 0 [0, oo)
GL_PACK_SKIP_ROWS integer 0 [0,oo)
GL_PACK_SKIP_PIXELS integer 0 [0,oo)
GL_PACK_SKIP_IMAGES 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_IMAGE_HEIGHT integer 0 [0,oo)
GL_UNPACK_SKIP_ROWS integer 0 [0,oo)
GL_UNPACK_SKIP_PIXELS integer 0 [0,oo)
GL_UNPACK_SKIP_IMAGES 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 _param2 is
0, the parameter is false; otherwise it is set to true.
If _param1 is a integer type parameter, _param2 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 _param2 is 0 and true otherwise.
NOTES
The pixel storage modes in effect when glDrawPixels,
glReadPixels, glTexImage1D, glTexImage2D, glTexImage3D,
glTexSubImage1D, glTexSubImage2D, glTexSubImage3D,
glBitmap, or glPolygonStipple is placed in a display list
control the interpretation of memory data. Likewise, if
the GL_ARB_imaging extension is supported, the pixel stor-
age modes in effect when glColorTable, glColorSubTable,
glConvolutionFilter1D, glConvolutionFilter2D, of
glSeparableFilter2D is placed in a display list control
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_IMAGE_HEIGHT
glGet with argument GL_PACK_SKIP_ROWS
glGet with argument GL_PACK_SKIP_PIXELS
glGet with argument GL_PACK_SKIP_IMAGES
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_IMAGE_HEIGHT
glGet with argument GL_UNPACK_SKIP_ROWS
glGet with argument GL_UNPACK_SKIP_PIXELS
glGet with argument GL_UNPACK_SKIP_IMAGES
glGet with argument GL_UNPACK_ALIGNMENT
SEE ALSO
glBitmap, glColorTable, glColorSubTable,
glConvolutionFilter1D, glConvolutionFilter2D,
glSeparableFilter2D, glDrawPixels, glHistogram, glMinmax,
glPixelMap, glPixelTransfer, glPixelZoom,
glPolygonStipple, glPushClientAttrib, glReadPixels,
glTexImage1D, glTexImage2D, glTexImage3D, glTexSubImage1D,
glTexSubImage2D, glTexSubImage3D
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