NAME

C SPECIFICATION

	GLfloat param )

	GLint param )

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

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 consists of bytes $b_0$ , $b_1$ , $b_2$ , $b_3$ , it is stored in memory as $b_3$ , $b_2$ , $b_1$ , $b_0$ 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 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

$k=\left\{\begin{array}{c}nl\\ \frac{a}{s}⌈\frac{snl}{a}⌉\end{array}\begin{array}{c}s\ge a\\ s<a\end{array}\right.$

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 $\mathrm{width}$ 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

$k=8a⌈\frac{nl}{8a}⌉$

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 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-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 $b_0$ , $b_1$ , $b_2$ , $b_3$ , it is taken from memory as $b_3$ , $b_2$ , $b_1$ , $b_0$ 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 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

$k=\left\{\begin{array}{c}nl\\ \frac{a}{s}⌈\frac{snl}{a}⌉\end{array}\begin{array}{c}s\ge a\\ s<a\end{array}\right.$

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 $\mathrm{width}$ 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

$k=8a⌈\frac{nl}{8a}⌉$

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 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 $in$ 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 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.

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

Pixel storage modes are client state and must be pushed and restored using
glPushClientAttrib and glPopClientAttrib.

ERRORS

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

SEE ALSO