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 3, 2015-02-17

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_TRANSPARENT_ALPHA_MESA                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_MESA then window
    surfaces created with the EGLConfig will have their alpha channel
    respected with pre-multiplied alpha implied. 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_MESA, 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_MESA 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 the current active display.

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

Issues

    1. Should non-window surfaces created with EGL_TRANSPARENT_TYPE
       set to EGL_TRANSPARENT_ALPHA_MESA 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_MESA will
       generate an error.

Revision History

    Version 3, 2015-02-17 (Jonny Lamb)
        - Added pre-multiplied alpha implication.

    Version 2, 2015-02-16 (Jonny Lamb)
        - Resolved 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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