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

[Nick Dyer]

On 27/05/2016 14:18, Hans Verkuil wrote:
> On 05/27/2016 02:52 PM, Nick Dyer wrote:
>> 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.
> 
> A V4L2_PIX_FMT_ definition must specify the format unambiguously. So it is not
> enough to just say that the data is a signed 16 bit value, you need to document
> exactly how it should be interpreted. Looking at the code it seems that the
> signed values are within a certain range and are normalized to 0-max by this line:
> 
> ssize_t gray = (data[i] - cfg->min) * max / (cfg->max - cfg->min);
> 
> Are the min/max values defined by the hardware? Because in that case this pixel
> format has to be a hardware-specific define (e.g. V4L2_PIX_FMT_FOO_S16).
> 
> Only if the min/max values are -32768 and 32767 can you really use YS16 (not sure
> yet about that name, but that's another issue).

I'm sorry, perhaps that is slightly misleading.

The data being output is the raw capacitance values from the analogue
front-end in the touch controller. Due to the physical characteristics,
there is a small range in the middle of the possible outputs of the ADC
which is interesting, and the heatmap tool allows mapping just that range
to the possible colors, otherwise everything would look mid-grey. So
heatmap is doing a job conceptually like adjusting the black point and
white point of a greyscale image to bring out the detail.

So the hardware itself doesn't have a conception of what those min/max
values are - from the hardware point of view the values may range from
-32768 to 32767 (in fact such values are commonly seen when one of the
touchscreen lines has a fault or is not connected).