glMapBufferRange — map a section of a buffer object's data store
void *glMapBufferRange( | GLenum target, |
| GLintptr offset, | |
| GLsizeiptr length, | |
GLbitfield access); |
targetSpecifies a binding to which the target buffer is bound.
offsetSpecifies the starting offset within the buffer of the range to be mapped.
lengthSpecifies the length of the range to be mapped.
accessSpecifies a combination of access flags indicating the desired access to the range.
GLboolean glUnmapBuffer( | GLenum target); |
glMapBufferRange maps all or part of the data store of a buffer object into the client's address
space. target specifies the target to which the buffer is bound and must be one of GL_ARRAY_BUFFER,
GL_COPY_READ_BUFFER, GL_COPY_WRITE_BUFFER, GL_ELEMENT_ARRAY_BUFFER,
GL_PIXEL_PACK_BUFFER, GL_PIXEL_UNPACK_BUFFER,
GL_TRANSFORM_FEEDBACK_BUFFER, or GL_UNIFORM_BUFFER. offset and
length indicate the range of data in the buffer object that is to be mapped, in terms of basic machine units.
access is a bitfield containing flags which describe the requested mapping. These flags are described below.
If no error occurs, a pointer to the beginning of the mapped range is returned once all pending operations on that buffer have
completed, and may be used to modify and/or query the corresponding range of the buffer, according to the following flag bits set
in access:
GL_MAP_READ_BIT indicates that the returned pointer may be used to read
buffer object data. No GL error is generated if the pointer is used to query
a mapping which excludes this flag, but the result is undefined and system
errors (possibly including program termination) may occur.
GL_MAP_WRITE_BIT indicates that the returned pointer may be used to modify
buffer object data. No GL error is generated if the pointer is used to modify
a mapping which excludes this flag, but the result is undefined and system
errors (possibly including program termination) may occur.
Furthermore, the following optional flag bits in access may be used to modify the mapping:
GL_MAP_INVALIDATE_RANGE_BIT indicates that the previous contents of the
specified range may be discarded. Data within this range are undefined with
the exception of subsequently written data. No GL error is generated if subsequent
GL operations access unwritten data, but the result is undefined and
system errors (possibly including program termination) may occur. This flag
may not be used in combination with GL_MAP_READ_BIT.
GL_MAP_INVALIDATE_BUFFER_BIT indicates that the previous contents of the
entire buffer may be discarded. Data within the entire buffer are undefined
with the exception of subsequently written data. No GL error is generated if
subsequent GL operations access unwritten data, but the result is undefined
and system errors (possibly including program termination) may occur. This
flag may not be used in combination with GL_MAP_READ_BIT.
GL_MAP_FLUSH_EXPLICIT_BIT indicates that one or more discrete subranges
of the mapping may be modified. When this flag is set, modifications to
each subrange must be explicitly flushed by calling glFlushMappedBufferRange.
No GL error is set if a subrange of the mapping is modified and
not flushed, but data within the corresponding subrange of the buffer are undefined.
This flag may only be used in conjunction with GL_MAP_WRITE_BIT.
When this option is selected, flushing is strictly limited to regions that are
explicitly indicated with calls to glFlushMappedBufferRange
prior to unmap; if this option is not selected glUnmapBuffer
will automatically flush the entire mapped range when called.
GL_MAP_UNSYNCHRONIZED_BIT indicates that the GL should not attempt to
synchronize pending operations on the buffer prior to returning from glMapBufferRange.
No GL error is generated if pending operations which source or modify the buffer overlap the mapped region,
but the result of such previous and any subsequent operations is undefined.
If an error occurs, glMapBufferRange returns a NULL pointer.
A mapped data store must be unmapped with glUnmapBuffer before its buffer object is used.
Otherwise an error will be generated by any GL command that attempts to dereference the buffer object's data store.
When a data store is unmapped, the pointer to its data store becomes invalid. glUnmapBuffer
returns GL_TRUE unless the data store contents have become corrupt during the time
the data store was mapped. This can occur for system-specific reasons that affect the availability of graphics
memory, such as screen mode changes. In such situations, GL_FALSE is returned and the
data store contents are undefined. An application must detect this rare condition and reinitialize the data store.
A buffer object's mapped data store is automatically unmapped when the buffer object is deleted or its data store
is recreated with glBufferData.
Mappings to the data stores of buffer objects may have nonstandard performance characteristics.
For example, such mappings may be marked as uncacheable regions of memory, and in such cases reading from them may be very slow.
To ensure optimal performance, the client should use the mapping in a fashion consistent
with the values of GL_BUFFER_USAGE and access.
Using a mapping in a fashion inconsistent with these values is liable to be multiple orders of magnitude slower
than using normal memory.
GL_INVALID_VALUE is generated if either of offset or length is negative,
or if offset + length is greater than the value of GL_BUFFER_SIZE.
GL_INVALID_VALUE is generated if access has any bits set other than those defined above.
GL_INVALID_OPERATION is generated for any of the following conditions:
The buffer is already in a mapped state.
Neither GL_MAP_READ_BIT or GL_MAP_WRITE_BIT is set.
GL_MAP_READ_BIT is set and any of GL_MAP_INVALIDATE_RANGE_BIT,
GL_MAP_INVALIDATE_BUFFER_BIT, or GL_MAP_UNSYNCHRONIZED_BIT is set.
GL_MAP_FLUSH_EXPLICIT_BIT is set and GL_MAP_WRITE_BIT is not set.
GL_OUT_OF_MEMORY is generated if glMapBufferRange fails because memory for the
mapping could not be obtained.
Copyright © 2010-2012 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999. http://opencontent.org/openpub/.