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.

Specifies the value that pname is set to.

If true, byte ordering for multibyte color components, depth components, or stencil indices is reversed. That is, if a four-byte component consists of bytes ${\mathit{b}}_0$, ${\mathit{b}}_1$, ${\mathit{b}}_2$, ${\mathit{b}}_3$, it is stored in memory as ${\mathit{b}}_3$, ${\mathit{b}}_2$, ${\mathit{b}}_1$, ${\mathit{b}}_0$ 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.

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.

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 $\mathit{p}$ in memory, then the location of the first pixel of the next row is obtained by skipping

components or indices, where $\mathit{n}$ is the number of components or indices in a pixel, $\mathit{l}$ is the number of pixels in a row (GL_PACK_ROW_LENGTH if it is greater than 0, the $\mathit{width}$ argument to the pixel routine otherwise), $\mathit{a}$ is the value of GL_PACK_ALIGNMENT, and $\mathit{s}$ is the size, in bytes, of a single component (if $\mathit{a}<\mathit{s}$, then it is as if $\mathit{a}=\mathit{s}$). In the case of 1-bit values, the location of the next row is obtained by skipping

$\mathit{k}=8\mathit{a}⌈\frac{\mathit{n}\mathit{l}}{8\mathit{a}}⌉$

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.

If greater than 0, GL_PACK_IMAGE_HEIGHT defines the number of pixels in an image three-dimensional 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 $\mathit{p}$ in memory, then the location of the first pixel of the next row is obtained by skipping

components or indices, where $\mathit{n}$ is the number of components or indices in a pixel, $\mathit{l}$ is the number of pixels in a row (GL_PACK_ROW_LENGTH if it is greater than 0, the $\mathit{width}$ argument to glTexImage3D otherwise), $\mathit{h}$ is the number of rows in a pixel image (GL_PACK_IMAGE_HEIGHT if it is greater than 0, the $\mathit{height}$ argument to the glTexImage3D routine otherwise), $\mathit{a}$ is the value of GL_PACK_ALIGNMENT, and $\mathit{s}$ is the size, in bytes, of a single component (if $\mathit{a}<\mathit{s}$, then it is as if $\mathit{a}=\mathit{s}$).

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.

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 $\mathit{i}$ is equivalent to incrementing the pointer by $\mathit{i}\mathit{n}$ components or indices, where $\mathit{n}$ is the number of components or indices in each pixel. Setting GL_PACK_SKIP_ROWS to $\mathit{j}$ is equivalent to incrementing the pointer by $\mathit{j}\mathit{m}$ components or indices, where $\mathit{m}$ 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 $\mathit{k}$ is equivalent to incrementing the pointer by $\mathit{k}\mathit{p}$, where $\mathit{p}$ is the number of components or indices per image, as computed in the GL_PACK_IMAGE_HEIGHT section.

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).

If true, byte ordering for multibyte color components, depth components, or stencil indices is reversed. That is, if a four-byte component consists of bytes ${\mathit{b}}_0$, ${\mathit{b}}_1$, ${\mathit{b}}_2$, ${\mathit{b}}_3$, it is taken from memory as ${\mathit{b}}_3$, ${\mathit{b}}_2$, ${\mathit{b}}_1$, ${\mathit{b}}_0$ 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.

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.

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 $\mathit{p}$ in memory, then the location of the first pixel of the next row is obtained by skipping

components or indices, where $\mathit{n}$ is the number of components or indices in a pixel, $\mathit{l}$ is the number of pixels in a row (GL_UNPACK_ROW_LENGTH if it is greater than 0, the $\mathit{width}$ argument to the pixel routine otherwise), $\mathit{a}$ is the value of GL_UNPACK_ALIGNMENT, and $\mathit{s}$ is the size, in bytes, of a single component (if $\mathit{a}<\mathit{s}$, then it is as if $\mathit{a}=\mathit{s}$). In the case of 1-bit values, the location of the next row is obtained by skipping

$\mathit{k}=8\mathit{a}⌈\frac{\mathit{n}\mathit{l}}{8\mathit{a}}⌉$

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.

If greater than 0, GL_UNPACK_IMAGE_HEIGHT defines the number of pixels in an image of a three-dimensional 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 $\mathit{p}$ in memory, then the location of the first pixel of the next row is obtained by skipping

components or indices, where $\mathit{n}$ is the number of components or indices in a pixel, $\mathit{l}$ is the number of pixels in a row (GL_UNPACK_ROW_LENGTH if it is greater than 0, the $\mathit{width}$ argument to glTexImage3D otherwise), $\mathit{h}$ is the number of rows in an image (GL_UNPACK_IMAGE_HEIGHT if it is greater than 0, the $\mathit{height}$ argument to glTexImage3D otherwise), $\mathit{a}$ is the value of GL_UNPACK_ALIGNMENT, and $\mathit{s}$ is the size, in bytes, of a single component (if $\mathit{a}<\mathit{s}$, then it is as if $\mathit{a}=\mathit{s}$).

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.

These values are provided as a convenience to the programmer; they provide no functionality that cannot be duplicated by incrementing the pointer passed to glTexImage1D, glTexImage2D, glTexSubImage1D or glTexSubImage2D. Setting GL_UNPACK_SKIP_PIXELS to $\mathit{i}$ is equivalent to incrementing the pointer by $\mathit{i}\mathit{n}$ components or indices, where $\mathit{n}$ is the number of components or indices in each pixel. Setting GL_UNPACK_SKIP_ROWS to $\mathit{j}$ is equivalent to incrementing the pointer by $\mathit{j}\mathit{k}$ components or indices, where $\mathit{k}$ is the number of components or indices per row, as just computed in the GL_UNPACK_ROW_LENGTH section.

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).