GLDRAWPIXELS()GLDRAWPIXELS()NAMEglDrawPixels - write a block of pixels to the frame buffer
C SPECIFICATION
void glDrawPixels( GLsizei width,
GLsizei height,
GLenum format,
GLenum type,
const GLvoid *pixels )
delim $$
PARAMETERS
width, height Specify the dimensions of the pixel rectangle to be writ‐
ten into the frame buffer.
format Specifies the format of the pixel data. Symbolic con‐
stants GL_COLOR_INDEX, GL_STENCIL_INDEX, GL_DEPTH_COMPO‐
NENT, 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. Symbolic 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.
DESCRIPTIONglDrawPixels reads pixel data from memory and writes it into the frame
buffer relative to the current raster position. Use glRasterPos to set
the current raster position; use glGet with argument GL_CUR‐
RENT_RASTER_POSITION to query the raster position.
Several parameters define the encoding of pixel data in memory and con‐
trol 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-pre‐
cision floating-point values, 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 format. Indices are
always treated individually. Color components are treated as groups of
one, two, three, or four values, again based on format. Both individ‐
ual 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 glPixelStore).
width$times$height pixels are read from memory, starting at location
pixels. By default, these pixels are taken from adjacent memory loca‐
tions, 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 param‐
eters specify different 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 oper‐
ated 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 val‐
ues. 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 neg‐
ative, 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 current raster position z coordi‐
nate 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 frag‐
ments 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 con‐
verted to fixed-point format, with an unspecified number of
bits to the right of the binary point, regardless of the mem‐
ory 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 references 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 scissor 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 posi‐
tive representable integer value maps to 1.0, and the most nega‐
tive 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 position 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 com‐
ponent is first, followed by green, followed by blue, followed
by alpha. Floating-point values are converted directly to an
internal floating-point format with unspecified precision.
Signed integer values are mapped linearly to the internal float‐
ing-point format such that the most positive representable inte‐
ger value maps to 1.0, and the most negative representable value
maps to -1.0. (Note that this mapping does not convert 0 pre‐
cisely 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 component is con‐
verted 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 component is con‐
verted 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 component is con‐
verted 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 component is con‐
verted 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 con‐
version, 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 component 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 con‐
version, 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 converted
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:
┌──────────────────┬────────────────────────────────────────┐
│ type │ corresponding 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 factors of 1. If
glPixelZoom is used to change the $x$ and $y$ pixel zoom factors, pix‐
els 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 col‐
umn 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_LUMI‐
NANCE_ALPHA, and the GL is in color index mode.
GL_INVALID_ENUM is generated if type is GL_BITMAP and format 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 executed between
the execution of glBegin and the corresponding 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, glPix‐
elMap, glPixelStore, glPixelTransfer, glPixelZoom, glRasterPos, glRead‐
Pixels, glScissor, glStencilFunc
GLDRAWPIXELS()
