openGL and JOGL

Reading, Writing, and Copying Pixel Data

OpenGL provides three basic commands that manipulate image data:

glReadPixels() -- Reads a rectangular array of pixels from the framebuffer and stores the data in processor memory.
glDrawPixels() -- Writes a rectangular array of pixels from data kept in processor memory into the framebuffer at the current raster position specified by glRasterPos*().
glCopyPixels() -- Copies a rectangular array of pixels from one part of the framebuffer to another. This command behaves similarly to a call to glReadPixels() followed by a call to glDrawPixels(), but the data is never written into processor memory.

void glReadPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid *pixels);

Reads pixel data from the framebuffer rectangle whose lower-left corner is at (x, y) and whose dimensions are width and height and stores it in the array pointed to by pixels. format indicates the kind of pixel data elements that are read (an index value or an R, G, B, or A component value, as listed in Table below), and type indicates the data type of each element shown below.

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

Draws a rectangle of pixel data with dimensions width and height. The pixel rectangle is drawn with its lower-left corner at the current raster position. format and type have the same meaning as with glReadPixels(). The array pointed to by pixels contains the pixel data to be drawn.

void glCopyPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum buffer);

Copies pixel data from the framebuffer rectangle whose lower-left corner is at (x, y) and whose dimensions are width and height. The data is copied to a new position whose lower-left corner is given by the current raster position. buffer is either GL_COLOR, GL_STENCIL, or GL_DEPTH, specifying the framebuffer that is used. glCopyPixels() behaves similarly to a glReadPixels() followed by a glDrawPixels(), with the following translation for the buffer to format parameter:
If buffer is GL_DEPTH or GL_STENCIL, then GL_DEPTH_COMPONENT or GL_STENCIL_INDEX is used, respectively.
If GL_COLOR is specified, GL_RGBA or GL_COLOR_INDEX is used, depending on whether the system is in RGBA or color-index mode.

JOGL: for JOGL the GLvoid * pixels Pointer becomes a type Buffer.

Pixel Formats for glReadPixels() or glDrawPixels():

format Constant	Pixel Format
GL_COLOR_INDEX	A single color index
GL_RGB	A red color component, followed by a green color component, followed by a blue color component
GL_RGBA	A red color component, followed by a green color component, followed by a blue color component, followed by an alpha color component
GL_RED	A single red color component
GL_GREEN	A single green color component
GL_BLUE	A single blue color component
GL_ALPHA	A single alpha color component
GL_LUMINANCE	A single luminance component
GL_LUMINANCE_ALPHA	A luminance component followed by an alpha color component
GL_STENCIL_INDEX	A single stencil index
GL_DEPTH_COMPONENT	A single depth component

Data Types for glReadPixels() or glDrawPixels():

type Constant

Data Type

GL_UNSIGNED_BYTE

unsigned 8-bit integer

GL_BYTE

signed 8-bit integer

GL_BITMAP

single bits in unsigned 8-bit integers using the same format as glBitmap()

GL_UNSIGNED_SHORT

unsigned 16-bit integer

GL_SHORT

signed 16-bit integer

GL_UNSIGNED_INT

unsigned 32-bit integer

GL_INT

signed 32-bit integer

GL_FLOAT

single-precision floating point

type Constant	Data Type
GL_UNSIGNED_BYTE	unsigned 8-bit integer
GL_BYTE	signed 8-bit integer
GL_BITMAP	single bits in unsigned 8-bit integers using the same format as `glBitmap()`
GL_UNSIGNED_SHORT	unsigned 16-bit integer
GL_SHORT	signed 16-bit integer
GL_UNSIGNED_INT	unsigned 32-bit integer
GL_INT	signed 32-bit integer
GL_FLOAT	single-precision floating point