GLDRAWPIXELS() MachTen Programmer’s Manual GLDRAWPIXELS()

NAME
glDrawPixels - write a block of pixels to the frame buffer

C SPECIFICATION
void glDrawPixels( GLsizei width,
GLsizei height,
GLenum format,
GLenum type,
const GLvoid *pixels )

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PARAMETERS
width, height Specify the dimensions of the pixel rectan-
gle to be written into the frame buffer.

format Specifies the format of the pixel data.
Symbolic constants GL_COLOR_INDEX,
GL_STENCIL_INDEX, GL_DEPTH_COMPONENT,
GL_RGBA, GL_RED, GL_GREEN, GL_BLUE,
GL_ALPHA, GL_RGB, GL_LUMINANCE, and
GL_LUMINANCE_ALPHA are accepted.

type Specifies the data type for pixels. Sym-
bolic constants GL_UNSIGNED_BYTE, GL_BYTE,
GL_BITMAP, GL_UNSIGNED_SHORT, GL_SHORT,
GL_UNSIGNED_INT, GL_INT, and GL_FLOAT are
accepted.

pixels Specifies a pointer to the pixel data.

DESCRIPTION
glDrawPixels reads pixel data from memory and writes it
into the frame buffer relative to the current raster posi-
tion. Use glRasterPos to set the current raster position;
use glGet with argument GL_CURRENT_RASTER_POSITION to
query the raster position.

Several parameters define the encoding of pixel data in
memory and control the processing of the pixel data before
it is placed in the frame buffer. These parameters are
set with four commands: glPixelStore, glPixelTransfer,
glPixelMap, and glPixelZoom. This reference page
describes the effects on glDrawPixels of many, but not
all, of the parameters specified by these four commands.

Data is read from pixels as a sequence of signed or
unsigned bytes, signed or unsigned shorts, signed or
unsigned integers, or single-precision floating-point val-
ues, depending on type. Each of these bytes, shorts,
integers, or floating-point values is interpreted as one
color or depth component, or one index, depending on for-
mat. Indices are always treated individually. Color com-
ponents are treated as groups of one, two, three, or four
values, again based on format. Both individual indices
and groups of components are referred to as pixels. If
type is GL_BITMAP, the data must be unsigned bytes, and
format must be either GL_COLOR_INDEX or GL_STENCIL_INDEX.
Each unsigned byte is treated as eight 1-bit pixels, with
bit ordering determined by GL_UNPACK_LSB_FIRST (see glPix-
elStore).

width$times$height pixels are read from memory, starting
at location pixels. By default, these pixels are taken
from adjacent memory locations, except that after all
width pixels are read, the read pointer is advanced to the
next four-byte boundary. The four-byte row alignment is
specified by glPixelStore with argument
GL_UNPACK_ALIGNMENT, and it can be set to one, two, four,
or eight bytes. Other pixel store parameters specify dif-
ferent read pointer advancements, both before the first
pixel is read and after all width pixels are read. See
the
glPixelStore reference page for details on these options.

The width$times$height pixels that are read from memory
are each operated on in the same way, based on the values
of several parameters specified by glPixelTransfer and
glPixelMap. The details of these operations, as well as
the target buffer into which the pixels are drawn, are
specific to the format of the pixels, as specified by for-
mat. format can assume one of eleven symbolic values:

GL_COLOR_INDEX
Each pixel is a single value, a color index. It
is converted to fixed-point format, with an
unspecified number of bits to the right of the
binary point, regardless of the memory data
type. Floating-point values convert to true
fixed-point values. Signed and unsigned integer
data is converted with all fraction bits set to
0. Bitmap data convert to either 0 or 1.

Each fixed-point index is then shifted left by
GL_INDEX_SHIFT bits and added to
GL_INDEX_OFFSET. If GL_INDEX_SHIFT is negative,
the shift is to the right. In either case, zero
bits fill otherwise unspecified bit locations in
the result.

If the GL is in RGBA mode, the resulting index
is converted to an RGBA pixel with the help of
the GL_PIXEL_MAP_I_TO_R, GL_PIXEL_MAP_I_TO_G,
GL_PIXEL_MAP_I_TO_B, and GL_PIXEL_MAP_I_TO_A
tables. If the GL is in color index mode, and
if GL_MAP_COLOR is true, the index is replaced
with the value that it references in lookup
table GL_PIXEL_MAP_I_TO_I. Whether the lookup
replacement of the index is done or not, the
integer part of the index is then ANDed with $2
sup b -1$, where $b$ is the number of bits in a
color index buffer.

The GL then converts the resulting indices or
RGBA colors to fragments by attaching the cur-
rent raster position z coordinate and texture
coordinates to each pixel, then assigning $x$
and $y$ window coordinates to the $n$th fragment
such that

$x sub n ~=~ x sub r ~+~ n ~ roman mod ~
"width"$

$y sub n ~=~ y sub r ~+~ | ~ n / "width" ~ |$

where ($x sub r , y sub r$) is the current
raster position. These pixel fragments are then
treated just like the fragments generated by
rasterizing points, lines, or polygons. Texture
mapping, fog, and all the fragment operations
are applied before the fragments are written to
the frame buffer.

GL_STENCIL_INDEX
Each pixel is a single value, a stencil index.
It is converted to fixed-point format, with an
unspecified number of bits to the right of the
binary point, regardless of the memory data
type. Floating-point values convert to true
fixed-point values. Signed and unsigned integer
data is converted with all fraction bits set to
0. Bitmap data convert to either 0 or 1.

Each fixed-point index is then shifted left by
GL_INDEX_SHIFT bits, and added to
GL_INDEX_OFFSET. If GL_INDEX_SHIFT is negative,
the shift is to the right. In either case, zero
bits fill otherwise unspecified bit locations in
the result. If GL_MAP_STENCIL is true, the
index is replaced with the value that it refer-
ences in lookup table GL_PIXEL_MAP_S_TO_S.
Whether the lookup replacement of the index is
done or not, the integer part of the index is
then ANDed with $2 sup b -1$, where $b$ is the
number of bits in the stencil buffer. The
resulting stencil indices are then written to
the stencil buffer such that the $n$th index is
written to location

$x sub n ~=~ x sub r ~+~ n ~ roman mod ~ "width"$

$y sub n ~=~ y sub r ~+~ | ~ n / "width" ~ |$

where ($x sub r , y sub r$) is the current raster
position. Only the pixel ownership test, the scis-
sor test, and the stencil writemask affect these
write operations.

GL_DEPTH_COMPONENT
Each pixel is a single-depth component. Floating-
point data is converted directly to an internal
floating-point format with unspecified precision.
Signed integer data is mapped linearly to the
internal floating-point format such that the most
positive representable integer value maps to 1.0,
and the most negative representable value maps to
-1.0. Unsigned integer data is mapped similarly:
the largest integer value maps to 1.0, and 0 maps
to 0.0. The resulting floating-point depth value
is then multiplied by by GL_DEPTH_SCALE and added
to GL_DEPTH_BIAS. The result is clamped to the
range [0,1].

The GL then converts the resulting depth components
to fragments by attaching the current raster posi-
tion color or color index and texture coordinates
to each pixel, then assigning $x$ and $y$ window
coordinates to the $n$th fragment such that

$x sub n ~=~ x sub r ~+~ n ~ roman mod ~ "width"$

$y sub n ~=~ y sub r ~+~ | ~ n / "width" ~ |$

where ($x sub r , y sub r$) is the current raster
position. These pixel fragments are then treated
just like the fragments generated by rasterizing
points, lines, or polygons. Texture mapping, fog,
and all the fragment operations are applied before
the fragments are written to the frame buffer.

GL_RGBA
Each pixel is a four-component group: for GL_RGBA,
the red component is first, followed by green, fol-
lowed by blue, followed by alpha. Floating-point
values are converted directly to an internal float-
ing-point format with unspecified precision.
Signed integer values are mapped linearly to the
internal floating-point format such that the most
positive representable integer value maps to 1.0,
and the most negative representable value maps to
-1.0. (Note that this mapping does not convert 0
precisely to 0.0.) Unsigned integer data is mapped
similarly: the largest integer value maps to 1.0,
and 0 maps to 0.0. The resulting floating-point
color values are then multiplied by GL_c_SCALE and
added to GL_c_BIAS, where c is RED, GREEN, BLUE,
and ALPHA for the respective color components. The
results are clamped to the range [0,1].

If GL_MAP_COLOR is true, each color component is
scaled by the size of lookup table
GL_PIXEL_MAP_c_TO_c, then replaced by the value
that it references in that table. c is R, G, B, or
A respectively.

The GL then converts the resulting RGBA colors to
fragments by attaching the current raster position
z coordinate and texture coordinates to each pixel,
then assigning $x$ and $y$ window coordinates to
the $n$th fragment such that

$x sub n ~=~ x sub r ~+~ n ~ roman mod ~ "width"$

$y sub n ~=~ y sub r ~+~ | ~ n / "width" ~ |$

where ($x sub r , y sub r$) is the current raster
position. These pixel fragments are then treated
just like the fragments generated by rasterizing
points, lines, or polygons. Texture mapping, fog,
and all the fragment operations are applied before
the fragments are written to the frame buffer.

GL_RED Each pixel is a single red component. This compo-
nent is converted to the internal floating-point
format in the same way the red component of an RGBA
pixel is. It is then converted to an RGBA pixel
with green and blue set to 0, and alpha set to 1.
After this conversion, the pixel is treated as if
it had been read as an RGBA pixel.

GL_GREEN
Each pixel is a single green component. This com-
ponent is converted to the internal floating-point
format in the same way the green component of an
RGBA pixel is. It is then converted to an RGBA
pixel with red and blue set to 0, and alpha set to
1. After this conversion, the pixel is treated as
if it had been read as an RGBA pixel.

GL_BLUE
Each pixel is a single blue component. This compo-
nent is converted to the internal floating-point
format in the same way the blue component of an
RGBA pixel is. It is then converted to an RGBA
pixel with red and green set to 0, and alpha set to
1. After this conversion, the pixel is treated as
if it had been read as an RGBA pixel.

GL_ALPHA
Each pixel is a single alpha component. This com-
ponent is converted to the internal floating-point
format in the same way the alpha component of an
RGBA pixel is. It is then converted to an RGBA
pixel with red, green, and blue set to 0. After
this conversion, the pixel is treated as if it had
been read as an RGBA pixel.

GL_RGB Each pixel is a three-component group: red first,
followed by green, followed by blue. Each compo-
nent is converted to the internal floating-point
format in the same way the red, green, and blue
components of an RGBA pixel are. The color triple
is converted to an RGBA pixel with alpha set to 1.
After this conversion, the pixel is treated as if
it had been read as an RGBA pixel.

GL_LUMINANCE
Each pixel is a single luminance component. This
component is converted to the internal floating-
point format in the same way the red component of
an RGBA pixel is. It is then converted to an RGBA
pixel with red, green, and blue set to the con-
verted luminance value, and alpha set to 1. After
this conversion, the pixel is treated as if it had
been read as an RGBA pixel.

GL_LUMINANCE_ALPHA
Each pixel is a two-component group: luminance
first, followed by alpha. The two components are
converted to the internal floating-point format in
the same way the red component of an RGBA pixel is.
They are then converted to an RGBA pixel with red,
green, and blue set to the converted luminance
value, and alpha set to the converted alpha value.
After this conversion, the pixel is treated as if
it had been read as an RGBA pixel.

The following table summarizes the meaning of the valid
constants for the type parameter:

center box ; ci | ci c | c . type corre-
sponding type = GL_UNSIGNED_BYTE unsigned 8-bit integer
GL_BYTE signed 8-bit integer
GL_BITMAP single bits in unsigned 8-bit integers
GL_UNSIGNED_SHORT unsigned 16-bit integer
GL_SHORT signed 16-bit integer
GL_UNSIGNED_INT unsigned 32-bit integer
GL_INT 32-bit integer
GL_FLOAT single-precision floating-point

The rasterization described so far assumes pixel zoom fac-
tors of 1. If
glPixelZoom is used to change the $x$ and $y$ pixel zoom
factors, pixels are converted to fragments as follows. If
($x sub r$, $y sub r$) is the current raster position, and
a given pixel is in the $n$th column and $m$th row of the
pixel rectangle, then fragments are generated for pixels
whose centers are in the rectangle with corners at

($x sub r + zoom sub x n$, $y sub r + zoom sub y
m$)

($x sub r + zoom sub x (n + 1)$, $y sub r + zoom
sub y ( m + 1 )$)

where $zoom sub x$ is the value of GL_ZOOM_X and $zoom sub
y$ is the value of GL_ZOOM_Y.

ERRORS
GL_INVALID_VALUE is generated if either width or height is
negative.

GL_INVALID_ENUM is generated if format or type is not one
of the accepted values.

GL_INVALID_OPERATION is generated if format is GL_RED,
GL_GREEN, GL_BLUE, GL_ALPHA, GL_RGB, GL_RGBA,
GL_LUMINANCE, or GL_LUMINANCE_ALPHA, and the GL is in
color index mode.

GL_INVALID_ENUM is generated if type is GL_BITMAP and for-
mat is not either GL_COLOR_INDEX or GL_STENCIL_INDEX.

GL_INVALID_OPERATION is generated if format is
GL_STENCIL_INDEX and there is no stencil buffer.

GL_INVALID_OPERATION is generated if glDrawPixels is exe-
cuted between the execution of glBegin and the correspond-
ing execution of glEnd.

ASSOCIATED GETS
glGet with argument GL_CURRENT_RASTER_POSITION
glGet with argument GL_CURRENT_RASTER_POSITION_VALID

SEE ALSO
glAlphaFunc, glBlendFunc, glCopyPixels, glDepthFunc,
glLogicOp, glPixelMap, glPixelStore, glPixelTransfer,
glPixelZoom, glRasterPos, glReadPixels, glScissor, glSten-
cilFunc

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