GLSELECTBUFFER() MachTen Programmer’s Manual GLSELECTBUFFER()

NAME
glSelectBuffer - establish a buffer for selection mode
values

C SPECIFICATION
void glSelectBuffer( GLsizei size,
GLuint *buffer )

PARAMETERS
size Specifies the size of buffer.

buffer Returns the selection data.

DESCRIPTION
glSelectBuffer 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 glInitNames, glLoadName, glPushName) when the render-
ing mode is GL_SELECT (see glRenderMode). glSelectBuffer
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 primi-
tives are drawn into some region of a window. The region
is defined by the current modelview and perspective matri-
ces.

In selection mode, no pixel fragments are produced from
rasterization. Instead, if a primitive or a raster posi-
tion intersects the clipping volume defined by the viewing
frustum and the user-defined clipping planes, this primi-
tive 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 glRenderMode is called, a hit
record is copied to buffer if any hits have occurred since
the last such event (name stack change or glRenderMode
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 con-
tents, 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.

To exit selection mode, call glRenderMode with an argument
other than GL_SELECT. Whenever glRenderMode 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
glRenderMode was called, a negative hit record count is
returned.

NOTES
The contents of buffer is undefined until glRenderMode is
called with an argument other than GL_SELECT.

glBegin/glEnd primitives and calls to glRasterPos can
result in hits.

ERRORS
GL_INVALID_VALUE is generated if size is negative.

GL_INVALID_OPERATION is generated if glSelectBuffer is
called while the render mode is GL_SELECT, or if glRender-
Mode is called with argument GL_SELECT before glSelect-
Buffer is called at least once.

GL_INVALID_OPERATION is generated if glSelectBuffer is
executed between the execution of glBegin and the corre-
sponding execution of glEnd.

ASSOCIATED GETS
glGet with argument GL_NAME_STACK_DEPTH

SEE ALSO
glFeedbackBuffer, glInitNames, glLoadName, glPushName,
glRenderMode

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