| Name |
| |
| ARM_pixmap_multisample_discard |
| |
| Name Strings |
| |
| EGL_ARM_pixmap_multisample_discard |
| |
| Contributors |
| |
| Arne Bergene Fossaa |
| Tom Cooksey |
| Endre Sund |
| David Garbett |
| |
| Contacts |
| |
| Tom Cooksey (tom 'dot' cooksey 'at' arm 'dot' com) |
| |
| Status |
| |
| Complete. |
| |
| Version |
| |
| Version 1, March 5, 2013 |
| |
| Number |
| |
| EGL Extension #54 |
| |
| Dependencies |
| |
| EGL 1.0 is required. |
| |
| This extension is written against the wording of the EGL 1.4 Specification. |
| |
| Overview |
| |
| ARM_pixmap_multisample_discard adds an attribute to eglCreatePixmapSurface |
| that allows the client API implementation to resolve a multisampled pixmap |
| surface, therefore allowing the multisample buffer to be discarded. |
| |
| Some GPU architectures - such as tile-based renderers - are capable of |
| performing multisampled rendering by storing multisample data in internal |
| high-speed memory and downsampling the data when writing out to external |
| memory after rendering has finished. Since per-sample data is never written |
| out to external memory, this approach saves bandwidth and storage space. In |
| this case multisample data gets discarded, however this is acceptable in |
| most cases. |
| |
| The extension provides the EGL_DISCARD_SAMPLES_ARM attribute that allows |
| for implicit resolution when rendering to a pixmap surface. This complements |
| the OpenGL ES EXT_multisampled_render_to_texture extension which provides |
| similar functionality for rendering to an OpenGL ES texture. |
| |
| New Types |
| |
| None |
| |
| New Procedures and Functions |
| |
| None |
| |
| New Tokens |
| |
| Accepted as an attribute name in the <attrib_list> argument of |
| eglCreatePixmapSurface and by the <attribute> parameter of eglQuerySurface: |
| |
| EGL_DISCARD_SAMPLES_ARM 0x3286 |
| |
| Changes to Chapter 3 of the EGL 1.4 Specification (EGL Functions and Errors) |
| |
| Modify the second paragraph under "The Multisample Buffer" of Section 3.4, |
| page 18 (Configuration Management) |
| |
| "Operations such as posting a surface with eglSwapBuffers (see section |
| 3.9.1, copying a surface with eglCopyBuffers (see section 3.9.2), reading |
| from the color buffer using client API commands, binding a client API |
| context to a surface (see section 3.7.3), and flushing to a pixmap surface |
| created with the EGL_DISCARD_SAMPLES_ARM attribute enabled (see |
| section 3.5.4) may cause resolution of the multisample buffer to the color |
| buffer." |
| |
| Modify the fifth paragraph under "The Multisample Buffer" of Section 3.4, |
| page 18 (Configuration Management) |
| |
| "There are no single-sample depth or stencil buffers for a multisample |
| EGLConfig, or with a pixmap surface created with the |
| EGL_DISCARD_SAMPLES_ARM attribute (see section 3.5.4). The only depth and |
| stencil buffers are those in the multisample buffer. If the color samples |
| in the multisample buffer store fewer bits than are stored in the color |
| buffers, this fact will not be reported accurately. Presumably a |
| compression scheme is being employed, and is expected to maintain an |
| aggregate resolution equal to that of the color buffers." |
| |
| Modify the fifth paragraph of Section 3.5.4, page 34 (Creating Native |
| Pixmap Rendering Surfaces) |
| |
| "attrib list specifies a list of attributes for the pixmap. The list has the |
| same structure as described for eglChooseConfig. Attributes that can be |
| specified in attrib list include EGL_VG_COLORSPACE, EGL_VG_ALPHA_FORMAT and |
| EGL_DISCARD_SAMPLES_ARM." |
| |
| Add the following between paragraphs eight and nine of Section 3.5.4, |
| page 34 (Creating Native Pixmap Rendering Surfaces) |
| |
| "EGL_DISCARD_SAMPLES_ARM specifies whether the client API implementation is |
| allowed to implicitly resolve the multisample buffer. On some GPU |
| architectures - such as tile-based renderers - an implicit resolve can avoid |
| writing the multisample buffer back to external memory as the multisample |
| data is stored in internal high-speed memory. |
| |
| The implicit resolve can occur when the client API uses the pixmap as the |
| source or destination of any operation, when flushing to the pixmap or when |
| the client API unbinds (or breaks) the pixmap. When these operations occur |
| is dependent on the client API implementation. They can occur as an explicit |
| part of client API functions (such as glFinish, glReadPixels and |
| glCopyTexImage) or they can occur implicitly. |
| |
| Further rendering causes the implementation to read the surface buffer and |
| any ancillary buffers back in as single-sampled data. |
| Therefore use of this attribute may result in lower quality images. |
| |
| Valid values are EGL_TRUE, in which case the multisample buffer can be |
| discarded, or EGL_FALSE, in which case the multisample buffer is preserved. |
| The default value is EGL_FALSE. |
| |
| Note that the multisample buffer may be discarded during eglMakeCurrent |
| regardless of the value of the EGL_DISCARD_SAMPLES_ARM attribute (see |
| section 3.7.3)." |
| |
| Modify the ninth paragraph of Section 3.5.4, page 34 (Creating Native |
| Pixmap Rendering Surfaces) |
| |
| "On failure eglCreatePixmapSurface returns EGL_NO_SURFACE. If the attributes |
| of pixmap do not correspond to config, then an EGL_BAD_MATCH error is |
| generated. If config does not support rendering to pixmaps (the |
| EGL_SURFACE_TYPE attribute does not contain EGL_PIXMAP_BIT), an |
| EGL_BAD_MATCH error is generated. If config does not support the colorspace |
| or alpha format attributes specified in attriblist (as defined for |
| eglCreateWindowSurface), an EGL_BAD_MATCH error is generated. If config does |
| not specify non-zero EGL_SAMPLES and EGL_SAMPLE_BUFFERS and the |
| EGL_DISCARD_SAMPLES_ARM attribute is set to EGL_TRUE, then an EGL_BAD_MATCH |
| error is generated. If config is not a valid EGLConfig, an EGL_BAD_CONFIG |
| error is generated. If pixmap is not a valid native pixmap handle, then an |
| EGL_BAD_NATIVE_PIXMAP error should be generated. If there is already an |
| EGLSurface associated with pixmap (as a result of a previous |
| eglCreatePixmapSurface call), then a EGL_BAD_ALLOC error is generated. |
| Finally, if the implementation cannot allocate resources for the new EGL |
| pixmap, an EGL_BAD_ALLOC error is generated." |
| |
| |
| Add the following entry to Table 3.5, page 36 |
| (Queryable surface attributes and types) |
| |
| Attribute Type Description |
| ------------------------- ------- --------------------------------------- |
| EGL_DISCARD_SAMPLES_ARM boolean Multisample resolve when flushing to |
| surface |
| |
| Add the following paragraph before the last paragraph of Section 3.5.7, |
| page 38 (Surface Attributes) |
| |
| "Querying EGL_DISCARD_SAMPLES_ARM returns whether a multisample resolve |
| is forced on every flush to the surface (see section 3.5.4). This will only |
| return EGL_TRUE for pixmap surfaces created with the EGL_DISCARD_SAMPLES_ARM |
| attribute set to EGL_TRUE. EGL_FALSE will be returned for window and |
| pbuffer surfaces." |
| |
| Issues |
| |
| 1. Should eglSurfaceAttrib accept EGL_DISCARD_SAMPLES_ARM? |
| RESOLVED: No. The attribute should be decided at surface creation time. |
| |
| 2. Should eglCreateWindowSurface or eglCreatePbufferSurface accept |
| EGL_DISCARD_SAMPLES_ARM? |
| RESOLVED: No. While the attribute could equally apply to window and |
| pbuffer surfaces, no use case has been identified to justify the |
| additional maintenance this would require. |
| |
| Revision History |
| |
| Version 1, 2013/03/05 - Original release. |
| |
