fglSelectBuffer(3G) OpenGL Reference fglSelectBuffer(3G)NAME
fglSelectBuffer - establish a buffer for selection mode values
FORTRAN SPECIFICATION
SUBROUTINE fglSelectBuffer( INTEGER*4 size,
CHARACTER*8 buffer )
PARAMETERS
size Specifies the size of buffer.
buffer Returns the selection data.
DESCRIPTION
fglSelectBuffer has two arguments: buffer is a pointer to an array of
unsigned integers, and size indicates the size of the array. buffer
returns values from the name stack (see fglInitNames, fglLoadName,
fglPushName) when the rendering mode is GL_SELECT (see fglRenderMode).
fglSelectBuffer must be issued before selection mode is enabled, and it
must not be issued while the rendering mode is GL_SELECT.
A programmer can use selection to determine which primitives are drawn
into some region of a window. The region is defined by the current
modelview and perspective matrices.
In selection mode, no pixel fragments are produced from rasterization.
Instead, if a primitive or a raster position intersects the clipping
volume defined by the viewing frustum and the user-defined clipping
planes, this primitive causes a selection hit. (With polygons, no hit
occurs if the polygon is culled.) When a change is made to the name
stack, or when fglRenderMode is called, a hit record is copied to buffer
if any hits have occurred since the last such event (name stack change or
fglRenderMode call). The hit record consists of the number of names in
the name stack at the time of the event, followed by the minimum and
maximum depth values of all vertices that hit since the previous event,
followed by the name stack contents, bottom name first.
Depth values (which are in the range [0,1]) are multiplied by 2^32 - 1,
before being placed in the hit record.
An internal index into buffer is reset to 0 whenever selection mode is
entered. Each time a hit record is copied into buffer, the index is
incremented to point to the cell just past the end of the block of names
- that is, to the next available cell. If the hit record is larger than
the number of remaining locations in buffer, as much data as can fit is
copied, and the overflow flag is set. If the name stack is empty when a
hit record is copied, that record consists of 0 followed by the minimum
and maximum depth values.
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fglSelectBuffer(3G) OpenGL Reference fglSelectBuffer(3G)
To exit selection mode, call fglRenderMode with an argument other than
GL_SELECT. Whenever fglRenderMode is called while the render mode is
GL_SELECT, it returns the number of hit records copied to buffer, resets
the overflow flag and the selection buffer pointer, and initializes the
name stack to be empty. If the overflow bit was set when fglRenderMode
was called, a negative hit record count is returned.
NOTES
The contents of buffer is undefined until fglRenderMode is called with an
argument other than GL_SELECT.
fglBegin/fglEnd primitives and calls to fglRasterPos can result in hits.
ERRORS
GL_INVALID_VALUE is generated if size is negative.
GL_INVALID_OPERATION is generated if fglSelectBuffer is called while the
render mode is GL_SELECT, or if fglRenderMode is called with argument
GL_SELECT before fglSelectBuffer is called at least once.
GL_INVALID_OPERATION is generated if fglSelectBuffer is executed between
the execution of fglBegin and the corresponding execution of fglEnd.
ASSOCIATED GETS
fglGet with argument GL_NAME_STACK_DEPTH
fglGet with argument GL_SELECTION_BUFFER_SIZE
fglGetPointerv with argument GL_SELECTION_BUFFER_POINTER
SEE ALSO
fglFeedbackBuffer, fglInitNames, fglLoadName, fglPushName, fglRenderMode
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