Re: [RFC] V4L2_PIX_FMT_MJPEG vs V4L2_PIX_FMT_JPEG

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

On Monday, 1 October 2018 19:28:58 EEST Hans Verkuil wrote:
> On 10/01/2018 06:12 PM, Ezequiel Garcia wrote:
> > On Mon, 2018-10-01 at 08:42 -0400, Nicolas Dufresne wrote:
> >> Le lundi 01 octobre 2018 à 10:43 +0200, Hans Verkuil a écrit :
> >>> It turns out that we have both JPEG and Motion-JPEG pixel formats
> >>> defined.
> >>> 
> >>> Furthermore, some drivers support one, some the other and some both.
> >>> 
> >>> These pixelformats both mean the same.
> >>> 
> >>> I propose that we settle on JPEG (since it seems to be used most often)
> >>> and add JPEG support to those drivers that currently only use MJPEG.
> >> 
> >> Thanks for looking into this. As per GStreamer code, I see 3 alias for
> >> JPEG. V4L2_PIX_FMT_MJPEG/JPEG/PJPG. I don't know the context, this code
> >> was written before I knew GStreamer existed. It's possible there is a
> >> subtle difference, I have never looked at it, but clearly all our JPEG
> >> decoder handle these as being the same.
> >> 
> >> https://cgit.freedesktop.org/gstreamer/gst-plugins-good/tree/sys/v4l2/gst
> >> v4l2object.c#n956
> > 
> > To add more data points on the gstreamer side, there's really no
> > difference between gstreamer's types image/jpeg and video/x-jpeg.
> > 
> > Notably, jpegdec element just stuffs a huffman table if one is missing,
> > for any jpeg:
> > 
> > https://cgit.freedesktop.org/gstreamer/gst-plugins-good/tree/ext/jpeg/gstj
> > pegdec.c#n584
> 
> lib/libv4lconvert/libv4lconvert.c also treats JPEG and MJPEG the same.
> 
> It looks like JPEG and MJPEG are randomly used and I don't think you can
> assume that one will have a huffman table and not the other.

That at least should be fixed. If we decide that whether the frames will 
contain a Huffman table or not is useful information for userspace, then we 
should convey it, either through the current mechanism (JPEG vs. MJPEG) or 
through a different mechanism. Otherwise, we can merge JPEG and MJPEG (as long 
as it doesn't break userspace).

-- 
Regards,

Laurent Pinchart