docs/specs/MESA_configless_context.spec

   1 Name
   2
   3     MESA_configless_context
   4
   5 Name Strings
   6
   7     EGL_MESA_configless_context
   8
   9 Contact
  10
  11     Neil Roberts <neil.s.roberts@intel.com>
  12
  13 Status
  14
  15     Superseded by the functionally identical EGL_KHR_no_config_context
  16     extension.
  17
  18 Version
  19
  20     Version 2, September 9, 2016
  21
  22 Number
  23
  24     EGL Extension #not assigned
  25
  26 Dependencies
  27
  28     Requires EGL 1.4 or later.  This extension is written against the
  29     wording of the EGL 1.4 specification.
  30
  31 Overview
  32
  33     This extension provides a means to use a single context to render to
  34     multiple surfaces which have different EGLConfigs. Without this extension
  35     the EGLConfig for every surface used by the context must be compatible
  36     with the one used by the context. The only way to render to surfaces with
  37     different formats would be to create multiple contexts but this is
  38     inefficient with modern GPUs where this restriction is unnecessary.
  39
  40 IP Status
  41
  42     Open-source; freely implementable.
  43
  44 New Procedures and Functions
  45
  46     None.
  47
  48 New Tokens
  49
  50     Accepted as <config> in eglCreateContext
  51
  52         EGL_NO_CONFIG_MESA                  ((EGLConfig)0)
  53
  54 Additions to the EGL Specification section "2.2 Rendering Contexts and Drawing
  55 Surfaces"
  56
  57     Add the following to the 3rd paragraph:
  58
  59    "EGLContexts can also optionally be created with respect to an EGLConfig
  60     depending on the parameters used at creation time. If a config is provided
  61     then additional restrictions apply on what surfaces can be used with the
  62     context."
  63
  64     Replace the last sentence of the 6th paragraph with:
  65
  66    "In order for a context to be compatible with a surface they both must have
  67     been created with respect to the same EGLDisplay. If the context was
  68     created without respect to an EGLConfig then there are no further
  69     constraints. Otherwise they are only compatible if:"
  70
  71     Remove the last bullet point in the list of constraints.
  72
  73 Additions to the EGL Specification section "3.7.1 Creating Rendering Contexts"
  74
  75     Replace the paragraph starting "If config is not a valid EGLConfig..."
  76     with
  77
  78    "The config argument can either be a valid EGLConfig or EGL_NO_CONFIG_MESA.
  79     If it is neither of these then an EGL_BAD_CONFIG error is generated. If a
  80     valid config is passed then the error will also be generated if the config
  81     does not support the requested client API (this includes requesting
  82     creation of an OpenGL ES 1.x context when the EGL_RENDERABLE_TYPE
  83     attribute of config does not contain EGL_OPENGL_ES_BIT, or creation of an
  84     OpenGL ES 2.x context when the attribute does not contain
  85     EGL_OPENGL_ES2_BIT).
  86
  87     Passing EGL_NO_CONFIG_MESA will create a configless context. When a
  88     configless context is used with the OpenGL API it can be assumed that the
  89     initial values of the context's state will be decided when the context is
  90     first made current. In particular this means that the decision of whether
  91     to use GL_BACK or GL_FRONT for the initial value of the first output in
  92     glDrawBuffers will be decided based on the config of the draw surface when
  93     it is first bound."
  94
  95 Additions to the EGL Specification section "3.7.3 Binding Contexts and
  96 Drawables"
  97
  98     Replace the first bullet point with the following:
  99
 100    "* If draw or read are not compatible with ctx as described in section 2.2,
 101       then an EGL_BAD_MATCH error is generated."
 102
 103     Add a second bullet point after that:
 104
 105    "* If draw and read are not compatible with each other as described in
 106       section 2.2, then an EGL_BAD_MATCH error is generated."
 107
 108 Issues
 109
 110     1.  What happens when an OpenGL context with a double-buffered surface and
 111         draw buffer set to GL_BACK is made current with a single-buffered
 112         surface?
 113
 114         NOT RESOLVED: There are a few options here.  An implementation can
 115         raise an error, change the drawbuffer state to GL_FRONT or just do
 116         nothing, expecting the application to set GL_FRONT drawbuffer before
 117         drawing.  However, this extension deliberately does not specify any
 118         required behavior in this corner case and applications should avoid
 119         mixing single- and double-buffered surfaces with configless contexts.
 120
 121         Future extensions may specify required behavior in this case.
 122
 123 Revision History
 124
 125     Version 2, September 9, 2016
 126         Defer to EGL_KHR_no_config_context (Adam Jackson)
 127
 128     Version 1, February 28, 2014
 129         Initial draft (Neil Roberts)