linux-kernel - Re: [PATCH v4] media: v4l2-ctrls: add control for dpcm predictor

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Message-ID: <CA+V-a8vaPwaivk7B8AQEUHWJHQ6LNZ=BwQEFsRyiek6HBBS28g@mail.gmail.com>
Date:	Tue, 18 Sep 2012 18:19:18 +0530
From:	Prabhakar Lad <prabhakar.csengg@...il.com>
To:	Sakari Ailus <sakari.ailus@....fi>
Cc:	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	dlos <davinci-linux-open-source@...ux.davincidsp.com>,
	Rob Landley <rob@...dley.net>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, Hans de Goede <hdegoede@...hat.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	LMML <linux-media@...r.kernel.org>
Subject: Re: [PATCH v4] media: v4l2-ctrls: add control for dpcm predictor

Hi Sakari,

On Thu, Sep 13, 2012 at 6:29 AM, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
> Hi Sakari,
>
> On Friday 07 September 2012 21:46:44 Sakari Ailus wrote:
>>
>> Could you replace the above with this text (with appropriate indentation
>> etc.) while keeping the reference to Wikipedia?
>>
>> ------8<------
>> Differential pulse-code modulation (DPCM) compression can be used to
>> compress the samples into fewer bits than they would otherwise require.
>> This is done by calculating the difference between consecutive samples
>> and outputting the difference which in average is much smaller than the
>> values of the samples themselves since there is generally lots of
>> correlation between adjacent pixels. In decompression the original
>> samples are reconstructed. The process isn't lossless as the encoded
>> sample size in bits is less than the original.
>>
>> Formats using DPCM compression include <xref
>> linkend="pixfmt-srggb10dpcm8" />.
>>
>> This control is used to select the predictor used to encode the samples.
>
> If I remember correctly this control will be used on the receiver side on
> DaVinci, to decode pixels not encode them. How is the predictor used in that
> case ? Must it match the predictor used on the encoding side ? If so I expect
> documentation to be available somewhere.
>
> The OMAP3 ISP supports both DPCM encoding and decoding, and documents the
> predictors as
>
> "- The simple predictor
>
> This predictor uses only the previous same color component value as a
> prediction value. Therefore, only two-pixel memory is required.
>
> - The advanced predictor
>
> This predictor uses four previous pixel values, when the prediction value is
> evaluated. This means that also the other color component values are used,
> when the prediction value has been defined."
>
> It also states the the simple predictor is preferred for 10-8-10 conversion,
> and the advanced predictor for 10-7-10 and 10-6-10 conversion.
>
What do you suggest ?

Regards,
--Prabhakar Lad

>> The main difference between the simple and the advanced predictors is
>> image quality, with advanced predictor supposed to produce better
>> quality images as a result. Simple predictor can be used e.g. for
>> testing purposes.
>> ------8<------
>
> --
> Regards,
>
> Laurent Pinchart
>
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