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