Name

    EGL_MESA_transparent_alpha

Name Strings

    EGL_MESA_transparent_alpha

Contributors

    Jonny Lamb <jonny.lamb@collabora.co.uk>

Contacts

    Jonny Lamb <jonny.lamb@collabora.co.uk>

Status

    Draft

Version

    Version 1, 2014-12-15

Number

    EGL Extension #XXX

Extension Type

    EGL display extension

Dependencies

    This extension is written against the wording of the 2014.08.17 revision
    of the EGL 1.5 Specification.

Overview

    This extension defines how to create EGL window surfaces which respect
    their alpha channels.

New Types

    None

New Procedures and Functions

    None

New Tokens

    Accepted as the value of the EGLConfig attribute EGL_TRANSPARENT_TYPE:

        EGL_MESA_TRANSPARENT_ALPHA                0xXXXX

Additions to the EGL 1.5 Specification

    In section 3.4 "Configuration Management", under the "Other EGLConfig
    Attribute Descriptions" heading, append to the paragraph which begins with
    "EGL_TRANSPARENT_TYPE indicates" and ends with "and
    EGL_TRANSPARENT_BLUE_VALUE, respectively" the following:

    "If the attribute is EGL_TRANSPARENT_ALPHA then window surfaces
    created with the EGLConfig will have their alpha channel
    respected. Creating non-window surfaces with the EGLConfig will
    result in an error."

    In the next paragraph replace the first sentence with:

    "If EGL_TRANSPARENT_TYPE is either EGL_NONE or EGL_TRANSPARENT_ALPHA, then
    the values for EGL_TRANSPARENT_RED_VALUE, EGL_TRANSPARENT_GREEN_VALUE, and
    EGL_TRANSPARENT_BLUE_VALUE are undefined."

    Before the paragraph which begins with "EGL_MAX_PBUFFER_WIDTH and
    EGL_MAX_PBUFFER_HEIGHT indicate" insert the following paragraph:

    "If EGL_TRANSPARENT_TYPE is EGL_TRANSPARENT_ALPHA and EGL_ALPHA_SIZE is 0,
    window surfaces created with the EGLConfig will be fully opaque, as if
    alpha were set to 1.0."

New Behavior

    To determine if the EGL implementation supports this extension, clients
    should query the EGL_EXTENSIONS string of EGL_NO_DISPLAY.

    To obtain an EGL window surface with a meaningful alpha channel, use an
    EGLConfig with EGL_TRANSPARENT_TYPE set to EGL_TRANSPARENT_ALPHA.

Issues

    1. Should non-window surfaces created with EGL_TRANSPARENT_TYPE set to
       EGL_TRANSPARENT_ALPHA generate an error or have no effect on the
       surface?

       RESOLUTION: Yes. Non-window surfaces created with
       EGL_TRANSPARENT_TYPE set to EGL_TRANSPARENT_ALPHA will generate
       an error.

Revision History

    Version 2, 2015-02-16 (Jonny Lamb)
        - Resolve issue #1 to "Yes".
        - Changed extension type to "display extension" and removed
          dependency on EGL_EXT_client_extensions.
        - Added MESA suffix to the new token.

    Version 1, 2014-12-15 (Jonny Lamb)
        - Initial draft

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