Re: RFC on proposed patches to mr97310a.c for gspca and v4l

[Thomas Kaiser <v4l@kaiser-linux.li>]

Hello Theodore

kilgota@banach.math.auburn.edu wrote:
>> For the brightness, I guess, 0 means dark and 0xff completely bright 
>> (sensor is in saturation)?
> 
> That of course is a guess. OTOH it could be on a scale of 0 to 0x80, or 
> it could be that only the digits 0 through 9 are actually used, and the 
> basis is then 100, or too many other variations to count. Also what is 
> considered a "normal" or an "average" value? The trouble with your 
> suggestion of a scale from 0 to 0xff is that it makes sense, and in a 
> situation like this one obviously can not assume that.

I don't really understand what you try to tell with this sentence:
"and in a situation like this one obviously can not assume that."

The values changed from 0x03 (dark) to 0xfc (bright), for me does this 
mean that the scale goes from 0x00 to 0xff!? Or I am wrong?

> 
> What I am suspecting is that these things have some kind of standard 
> definitions, which are not necessarily done by logic but by convention, 
> and there is a document out there somewhere which lays it all down. The 
> document could have been produced by Microsoft, for example, which 
> doubtless has its own problems reducing chaos to order in the industry, 
> or by some kind of consortium of camera manufacturers, or something like 
> that. I really do strongly suspect that the interpretation of all of 
> this is written down somewhere. But I don't know where to look.

I believe that this documents are exists, but not available for 
public:-( Just company confidential.

Anyway most of the Linux webcam drivers were done by re-engineering the 
Windoz driver (usbsnoop). That said, all information about the cams is 
"a guess".

For the brightness thing, I just was working with a light and studied 
what is changing in the header of the frame. At this time I did this, I 
was not aware that I could remove the lens of the webcam to be more 
sensible to light change and get more precise results.

During the work I did for the PAC7311 Pixart chip I found out that 
removing the lens and put light directly to the sensor does help a lot 
to figure out how the cam is working.

And with this idea in mind, we could even get further to guess the 
compression algo from a cam.

Assuming that the sensor has a Bayer pattern.
- remove lens.
- put white light on the sensor
- use color filter an put each spectrum (RGB) on the sensor
- check the stream and find out what is changing in the stream according 
to the different light conditions.

Looks like I get off topic, now ;-)

Something else comes in my mind. Would it good to document all this what 
we are talking bout somewhere on a webpage?

Thomas