linux-kernel - Re: [PATCH v2 2/8] [media] Add signed 16-bit pixel format

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Message-ID: <82b68931-0da1-bd26-87c1-1cd9e2296f71@itdev.co.uk>
Date:	Fri, 27 May 2016 13:52:13 +0100
From:	Nick Dyer <nick.dyer@...ev.co.uk>
To:	Hans Verkuil <hverkuil@...all.nl>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org,
	Benjamin Tissoires <benjamin.tissoires@...hat.com>,
	Benson Leung <bleung@...omium.org>,
	Alan Bowens <Alan.Bowens@...el.com>,
	Javier Martinez Canillas <javier@....samsung.com>,
	Chris Healy <cphealy@...il.com>,
	Henrik Rydberg <rydberg@...math.org>,
	Andrew Duggan <aduggan@...aptics.com>,
	James Chen <james.chen@....com.tw>,
	Dudley Du <dudl@...ress.com>,
	Andrew de los Reyes <adlr@...omium.org>,
	sheckylin@...omium.org, Peter Hutterer <peter.hutterer@...-t.net>,
	Florian Echtler <floe@...terbrot.org>, mchehab@....samsung.com
Subject: Re: [PATCH v2 2/8] [media] Add signed 16-bit pixel format

On 27/05/2016 13:38, Hans Verkuil wrote:
> On 05/04/2016 07:07 PM, Nick Dyer wrote:
>> +    <refname><constant>V4L2_PIX_FMT_YS16</constant></refname>
>> +    <refpurpose>Grey-scale image</refpurpose>
>> +  </refnamediv>
>> +  <refsect1>
>> +    <title>Description</title>
>> +
>> +    <para>This is a signed grey-scale image with a depth of 16 bits per
>> +pixel. The most significant byte is stored at higher memory addresses
>> +(little-endian).</para>
> 
> I'm not sure this should be described in terms of grey-scale, since negative
> values make no sense for that. How are these values supposed to be interpreted
> if you want to display them? -32768 == black and 32767 is white?

We have written a utility to display this data and it is able to display
the values mapped to grayscale or color:
https://github.com/ndyer/heatmap/blob/master/src/display.c#L44

An example of the output is here:
https://www.youtube.com/watch?v=Uj4T6fUCySw

The data is intrinsically signed because that's how the low level touch
controller treats it. I'm happy to change it to "Signed image" if you think
that would be better.