by Laurent Pinchart

Hi Laurent,

On Thu, Feb 22, 2018 at 02:47:06PM +0200, Laurent Pinchart wrote:
> Hi Jacopo,
>
> On Thursday, 22 February 2018 14:36:00 EET jacopo mondi wrote:
> > On Thu, Feb 22, 2018 at 02:14:53PM +0200, Laurent Pinchart wrote:
> > > On Thursday, 22 February 2018 14:04:12 EET jacopo mondi wrote:
> > >> On Wed, Feb 21, 2018 at 10:28:06PM +0200, Laurent Pinchart wrote:
> > >>> On Tuesday, 20 February 2018 10:58:57 EET jacopo mondi wrote:
>
> [snip]
>
> > >>>> This actually makes me wonder if those informations (ycbcb_enc,
> > >>>> quantization and xfer_func) shouldn't actually be part of the
> > >>>> supported format list... I blindly added those default fields in the
> > >>>> try_fmt function, but I doubt they applies to all supported formats.
> > >>>>
> > >>>> Eg. the sensor supports YUYV as well as 2 RGB encodings (RGB444 and
> > >>>> RGB 565) and 1 raw format (BGGR). I now have a question here:
> > >>>>
> > >>>> 1) ycbcr_enc transforms non-linear R'G'B' to Y'CbCr: does this
> > >>>> applies to RGB and raw formats? I don't think so, and what value is
> > >>>> the correct one for the ycbcr_enc field in this case? I assume
> > >>>> xfer_func and quantization applies to all formats instead..
> > >>>
> > >>> There's no encoding for RGB formats if I understand things correctly,
> > >>> so I'd set ycbcr_enc to V4L2_YCBCR_ENC_DEFAULT. The transfer function
> > >>> and the quantization apply to all formats, but I'd be surprised to find
> > >>> a sensor outputting limited range for RGB.
> > >>
> > >> Ack, we got the same understanding for RGB formats. I wonder if for
> > >> those formats we wouldn't need a V4L2_YCBCR_ENC_NONE or similar...
> > >
> > > That, or explicitly documenting that when the format is not YUV the field
> > > should be set by both drivers and applications to V4L2_YCBCR_ENC_DEFAULT
> > > when written and ignored when read.
> >
> > Well, if no encoding is performed because the color encoding scheme is
> > RGB, the colorspace does anyway define an encoding method, so it seems
> > to me the latter is more appropriate (use DEFAULT and ignore when read).
> >
> > That's anyway just my opinion, but I could send a patch for
> > documentation and see what feedback it gets.
> >
> > >>> Have you been able to check whether the sensor outputs limited range
> > >>> of full range YUV ? If it outputs full range you can hardcode
> > >>> quantization to V4L2_QUANTIZATION_FULL_RANGE for all formats.
> > >>
> > >> In YUYV mode, I see values > 0xf0 ( > 240, which is the max value for
> > >> CbCr samples in limited quantization range), so I assume quantization
> > >> is full range.
> > >
> > > It should be, yes. What's the minimum and maximum values you get ?
> >
> > From a white surface:
> > min = 0x39
> > max = 0xfc
> >
> > From a black surface:
> > min = 0x00 (with 62 occurrences)
> > max = 0x8b
> >
> > I guess that's indeed full range quantization
>
> Could you check Y and UV separately ?
>
> #!/usr/bin/python
>
> import sys
>
> def main(argv):
> if len(argv) != 2:
> print('Usage: %s <file>' % argv[0])
> return 1
>
> data = open(argv[1], 'rb').read()
>
> y_min = 255
> y_max = 0
> uv_min = 255
> uv_max = 0
>
> for i in range(len(data) // 2):
> y = data[2*i]
> uv = data[2*i]

uv = data[2*i+1]

>
> y_min = min(y_min, y)
> y_max = max(y_max, y)
> uv_min = min(uv_min, uv)
> uv_max = max(uv_max, uv)
>
> print('Y [%u, %u] UV [%u, %u]' % (y_min, y_max, uv_min, uv_max))
> return 0
>
> if __name__ == '__main__':
> sys.exit(main(sys.argv))
>

White image:
Y [57, 252] UV [105, 145]

Black image:
Y [0, 32] UV [116, 139]