Name
NV_post_sub_buffer
Name Strings
EGL_NV_post_sub_buffer
Contributors
Arcady Goldmints-Orlov
James Jones
Daniel Kartch
Contact
James Jones, NVIDIA Corporation (jajones 'at' nvidia.com)
Status
Draft.
Version
Version 3, November 5, 2010
Number
EGL Extension #27
Dependencies
Requires EGL 1.1
This extension is written against the wording of the EGL 1.4
Specification
Overview
Many EGL client APIs do not support rendering to window surfaces
directly, so there is no way to efficiently make small updates to
window surfaces. Applications that need to perform many small updates
must either use the back-buffer preservation flag which forces
eglSwapBuffers to copy the entire back surface, or redraw the entire
back buffer on every update and hope eglSwapBuffers is implemented
using buffer-flipping. This extension provides a 3rd alternative: a
function which posts a sub-rectangle of a window surface and
preserves the back-buffer contents.
New Types
None.
New Procedures and Functions
EGLBoolean eglPostSubBufferNV(EGLDisplay dpy,
EGLSurface surface,
EGLint x, EGLint y,
EGLint width, EGLint height);
New Tokens
Accepted by the <attribute> parameter of eglQuerySurface and by the
<attrib_list> parameter of eglCreateWindowSurface:
EGL_POST_SUB_BUFFER_SUPPORTED_NV 0x30BE
Changes to Chapter 3 of the EGL 1.4 Specification (EGL Functions and Errors)
Modify the second paragraph of Section 3.5.1, page 27
(Creating On-Screen Rendering Surfaces)
"<attrib_list> specifies a list of attributes for the window. The list
has the same structure as described for eglChooseConfig. Attributes
that can be specified in <attrib_list> include EGL_POST_SUB_BUFFER_-
SUPPORTED_NV, EGL_RENDER_BUFFER, EGL_VG_COLORSPACE, and EGL_VG_ALPHA_-
FORMAT."
Add the following between paragraphs 4 and 5 of Section 3.5.1, page 27
(Creating On-Screen Rendering Surfaces)
"EGL_POST_SUB_BUFFER_SUPPORTED_NV specifies whether the application
would perfer a surface that supports sub-buffer post operations, as
described in section 3.9.1. Its values can be EGL_TRUE, in which case
the implementation will attempt to allocate a surface that supports
sub-buffer posts, or EGL_FALSE, in which case the implementation will
not take sub-buffer post capabilities into account.
"Implementations may not be able to support sub-buffer post
mechanisms, or may support them only on some native windows. Use
eglQuerySurface to determine a surface's capabilities (see section
3.5.6)."
Add the following entry to Table 3.5, page 36
(Queryable surface attributes and types)
Attribute Type Description
-------------------------------- ------- ------------------------
EGL_POST_SUB_BUFFER_SUPPORTED_NV boolean Surface can be used with
eglPostSubBufferNV
Add the following paragraph to Section 3.5.6, page 37
(Surface Attributes)
"Querying EGL_POST_SUB_BUFFER_SUPPORTED_NV returns EGL_TRUE if the
surface can use eglPostSubBufferNV (See section 3.9.1) to post sub-
rectangles of the back color buffer. Otherwise, EGL_FALSE is
returned."
Replace all but the last paragraph of section Section 3.9.1, page 50
(Posting to a Window)
"To post the color buffer to a window, call
EGLBoolean eglSwapBuffers(EGLDisplay dpy,
EGLSurface surface);
"To post a sub-rectangle of the color buffer to a window, call
EGLBoolean eglPostSubBufferNV(EGLDisplay dpy,
EGLSurface surface, EGLint x, EGLint y,
EGLint width, EGLint height);
"Where <x> and <y> are pixel offsets from the bottom-left corner of
<surface>.
"If <surface> is a back-buffered surface, then the requested portion
of the color buffer is copied to the native window associated with
that surface. If <surface> is a single-buffered window, pixmap, or
pbuffer surface, eglSwapBuffers and eglPostSubBufferNV have no
effect.
"The contents of ancillary buffers are always undefined after calling
eglSwapBuffers or eglPostSubBufferNV. The contents of the color
buffer are unchanged if eglPostSubBufferNV is called, or if
eglSwapBuffers is called and the value of the EGL_SWAP_BEHAVIOR
attribute of <surface> is EGL_BUFFER_PRESERVED. The value of EGL_-
SWAP_BEHAVIOR can be set for some surfaces using eglSurfaceAttrib, as
described in section 3.5.6.
"Native Window Resizing
"If the native window corresponding to <surface> has been resized
prior to the swap, <surface> must be resized to match. <surface> will
normally be resized by the EGL implementation at the time the native
window is resized. If the implementation cannot do this transparently
to the client, then eglSwapBuffers and eglPostSubBufferNV must
detect the change and resize <surface> prior to copying its pixels to
the native window. The sub-rectangle defined by <x>, <y>, <width>, and
<height> parameters to eglPostSubBufferNV will be clamped to the
extents of <surface>. If, after clamping, the rectangle contains no
pixels, eglPostSubBufferNV will have no effect."
Modify the following sentences in Section 3.9.3, page 51 (Posting
Semantics)
Paragraph 2, first sentence:
"If <dpy> and <surface> are the display and surface for the calling
thread's current context, eglSwapBuffers, eglPostSubBufferNV, and
eglCopyBuffers perform an implicit flush operation on the context
(glFlush for OpenGL or OpenGL ES context, vgFlush for an OpenVG
context)."
Paragraph 3, first sentence:
"The destination of a posting operation (a visible window, for
eglSwapBuffers or eglPostSubBufferNV, or a native pixmap, for
eglCopyBuffers) should have the same number of components and
component sizes as the color buffer it's being copied from."
Paragraph 6, first two sentences:
"The function
EGLBoolean eglSwapInterval(EGLDisplay dpy, EGLint
interval);
specifes the minimum number of video frame periods per color buffer
post operation for the window associated with the current context. The
interval takes effect when eglSwapBuffers or eglPostSubBufferNV is
first called subsequent to the eglSwapInterval call."
Modify the following sentences in Section 3.9.4, page 52 (Posting
Errors)
Paragraph 1, first sentence:
"eglSwapBuffers, eglPostSubBufferNV, and eglCopyBuffers return
EGL_FALSE on failure."
Paragraph 1, seventh sentence:
"If eglSwapBuffers or eglPostSubBufferNV are called and the native
window associated with <surface> is no longer valid, an EGL_BAD_-
NATIVE_WINDOW error is generated. If eglPostSubBufferNV is called
and <x>, <y>, <width>, or <height> are less than zero, EGL_BAD_-
PARAMETER is generated."
Issues
1. Should all surfaces be required to support sub-buffer posts if
this extension is supported?
RESOLVED: No. Some implementations may support multiple types of
native windows. Support for sub-surface posting is therefore a
per-surface property, so a surface query should be used to determine
which surfaces support sub-surface posts.
2. What should this extension be called?
RESOLVED: Names considered EGL_NV_copy_sub_buffer, EGL_NV_present_sub-
surface, EGL_NV_post_sub_buffer. eglCopySubBuffer() sounded too
similar to eglCopyBuffer(), which operates on different types of
surfaces. EGL_present_sub_surface was originally chosen as it was
sufficiently different than eglCopyBuffer(), but based on internal
feedback, the term "Post" is preferable to "Present" because it is
already used in the EGL spec to describe buffer presentation
operations. "Buffer" was chosen over "surface" at this point as well,
because it is more consistent with the eglSwapBuffers() and
eglCopyBuffer() commands, and eglPostSubBuffer() is still
differentiated enough from eglCopyBuffer() that the two won't be
confused.
Revision History
#3 (James Jones, November 5, 2010)
-Renamed from NV_present_sub_surface to NV_post_sub_buffer based on
feedback from internal reviews.
-Allowed EGL_POST_SUB_BUFFER_SUPPORTED_NV to be used as a hint when
creating window surfaces.
-Clarified that eglSwapInterval applies to all color-buffer post
operations affecting on-screen surfaces, not just eglSwapBuffers.
#2 (James Jones, November 1, 2010)
- Fixed a few typos.
#1 (James Jones, October 22, 2010)
- Initial revision, based on GLX_MESA_copy_sub_buffer
