[Bug 783829] New: omxh264enc: unclear 'interval-intraframes' property

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=783829

            Bug ID: 783829
           Summary: omxh264enc: unclear 'interval-intraframes' property
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-omx
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: gdesmott at gnome.org
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

The omxh264enc element has this property:

  interval-intraframes: Interval of coding Intra frames (0xffffffff=component
default)

If set, it will override the OMX_VIDEO_CONFIG_AVCINTRAPERIOD.nPFrames setting
(section 4.3.27 in the spec,
https://www.khronos.org/registry/OpenMAX-IL/specs/OpenMAX_IL_1_1_2_Specification.pdf
) which is defined as:
"nPFrames specifies coding of a frame as Intra (non-inclusive of the first
frame)
after every nPFrames of Inter frames."
So the number of P frames between each I frame basically.

A couple of things aren't clear:

- Why is it setting OMX_VIDEO_CONFIG_AVCINTRAPERIOD.nPFrames and not
OMX_VIDEO_PARAM_AVCTYPE.nPFrames? This setting is defined in the spec (4.3.18)
as:
"nPFrames is the number of P frames between I frames."
The OMX spec isn't very clear about the difference between these two settings
actually. If someone knows more about it please let me know.

- This definition ("Interval of coding Intra frames") doesn't account for
potential B frames. The interval between 2 I frame is actually nPFrames +
nBFrames.

We'd need a gst setting to set nBFrames as well so we could make things clearer
for the 'interval-intraframes' property in the meantime. But first we should
clarify the OMX_VIDEO_CONFIG_AVCINTRAPERIOD.nPFrames vs
OMX_VIDEO_CONFIG_AVCINTRAPERIOD.nPFrames distinction.

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