linux-kernel - Re: [PATCH 1/6] staging: Import the BCM2835 MMAL-based V4L2 camera driver.

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Message-ID: <f6506c26-f5ad-e7a9-3883-1fc40101ccf0@xs4all.nl>
Date:   Fri, 10 Feb 2017 10:47:23 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Dave Stevenson <linux-media@...tevenson.freeserve.co.uk>,
        Eric Anholt <eric@...olt.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-rpi-kernel@...ts.infradead.org, linux-media@...r.kernel.org
Subject: Re: [PATCH 1/6] staging: Import the BCM2835 MMAL-based V4L2 camera
 driver.

On 02/06/2017 05:00 PM, Hans Verkuil wrote:
> On 02/06/2017 04:21 PM, Dave Stevenson wrote:
>> Hi Hans.
>>
>> On 06/02/17 12:58, Hans Verkuil wrote:
>>> On 02/06/2017 12:37 PM, Dave Stevenson wrote:
>>>> Hi Hans.
>>>>
>>>> On 06/02/17 09:08, Hans Verkuil wrote:
>>>>> Hi Eric,
>>>>>
>>>>> Great to see this driver appearing for upstream merging!
>>>>>
>>>>> See below for my review comments, focusing mostly on V4L2 specifics.
>>>>>
>>
>> <snip>
>>
>>>>>> +	f->fmt.pix.pixelformat = dev->capture.fmt->fourcc;
>>>>>> +	f->fmt.pix.bytesperline = dev->capture.stride;
>>>>>> +	f->fmt.pix.sizeimage = dev->capture.buffersize;
>>>>>> +
>>>>>> +	if (dev->capture.fmt->fourcc == V4L2_PIX_FMT_RGB24)
>>>>>> +		f->fmt.pix.colorspace = V4L2_COLORSPACE_SRGB;
>>>>>> +	else if (dev->capture.fmt->fourcc == V4L2_PIX_FMT_JPEG)
>>>>>> +		f->fmt.pix.colorspace = V4L2_COLORSPACE_JPEG;
>>>>>> +	else
>>>>>> +		f->fmt.pix.colorspace = V4L2_COLORSPACE_SMPTE170M;
>>>>>
>>>>> Colorspace has nothing to do with the pixel format. It should come from the
>>>>> sensor/video receiver.
>>>>>
>>>>> If this information is not available, then COLORSPACE_SRGB is generally a
>>>>> good fallback.
>>>>
>>>> I would if I could, but then I fail v4l2-compliance on V4L2_PIX_FMT_JPEG
>>>> https://git.linuxtv.org/v4l-utils.git/tree/utils/v4l2-compliance/v4l2-test-formats.cpp#n329
>>>> The special case for JPEG therefore has to remain.
>>>
>>> Correct. Sorry, my fault, I forgot about that.
>>>
>>>>
>>>> It looks like I tripped over the subtlety between V4L2_COLORSPACE_,
>>>> V4L2_XFER_FUNC_, V4L2_YCBCR_ENC_, and V4L2_QUANTIZATION_, and Y'CbCr
>>>> encoding vs colourspace.
>>>>
>>>> The ISP coefficients are set up for BT601 limited range, and any
>>>> conversion back to RGB is done based on that. That seemed to fit
>>>> SMPTE170M rather than SRGB.
>>>
>>> Colorspace refers to the primary colors + whitepoint that are used to
>>> create the colors (basically this answers the question to which colors
>>> R, G and B exactly refer to). The SMPTE170M has different primaries
>>> compared to sRGB (and a different default transfer function as well).
>>>
>>> RGB vs Y'CbCr is just an encoding and it doesn't change the underlying
>>> colorspace. Unfortunately, the term 'colorspace' is often abused to just
>>> refer to RGB vs Y'CbCr.
>>>
>>> If the colorspace is SRGB, then when the pixelformat is a Y'CbCr encoding,
>>> then the BT601 limited range encoding is implied, unless overridden via
>>> the ycbcr_enc and/or quantization fields in struct v4l2_pix_format.
>>>
>>> In other words, this does already the right thing.
>>
>> https://linuxtv.org/downloads/v4l-dvb-apis-new/uapi/v4l/pixfmt-007.html#colorspace-srgb-v4l2-colorspace-srgb
>> "The default transfer function is V4L2_XFER_FUNC_SRGB. The default 
>> Y’CbCr encoding is V4L2_YCBCR_ENC_601. The default Y’CbCr quantization 
>> is full range."
>> So full range or limited?
> 
> Ah, good catch. The default range for SRGB is full range, so the documentation
> is correct. This is according to the sYCC standard.
> 
> This means that you need to set the quantization field to limited range in this driver.
> 
> Sorry for the confusion I caused.
> 
> Interesting, I should take a look at other drivers since I suspect that this is
> signaled wrong elsewhere as well. It used to be limited range but I changed it
> to full range (as per the sYCC spec). But in practice it is limited range in most
> cases.
> 
> I'll take another look at this on Friday.
> 
> I recommend that you leave the code as is for now.

I posted a patch to correct the default quantization range mapping for sRGB and AdobeRGB:

https://patchwork.linuxtv.org/patch/39306/

So even though the standards say full range, this would break backwards compatibility
with all current kernel drivers since they all produce limited range when converting
sRGB or AdobeRGB to a Y'CbCr format.

Thanks for bringing this to my attention!

Regards,

	Hans