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

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

kilgota@banach.math.auburn.edu wrote:
>>> 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."
> 
> I mean, your interpretation of 0 to 0xff is a natural and sensible 
> interpretation (for us). But what one can not assume is, it made sense 
> to those who constructed the system. Perhaps those guys were setting it 
> all up differently.

You are right, we don't know what the developer were thinking, 
unfortunately,

You have to turn yourself in a webcam developer and think how you would 
do it. When you find, by observing/testing the cam, that it looks 
similar as you thought about how to do it, the observation should be true!

> 
>>
>> 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?
> 
> Well, if you have actual data to back up your impressions about this, 
> then clearly you have evidence. So that is good, obviously.

I will do this again in the next couple of weeks (lens removed).

> 
>>
>>>
>>> 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.
> 
> That may be true. If so, then such documentation is indeed not 
> available. But sometimes a document is published and available, and one 
> just is not aware of the fact.
> 
>>
>> 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".
> 
> Very true. Also true about the still cameras that I supported in 
> libgphoto2. There are no secrets kept on the USB bus. But what is done 
> inside the computer does not appear on the USB bus and there is no log 
> of it.
> 
>>
>> 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 ;-)
> 
> But it is very interesting nevertheless.

I think so, I didn't try with the color filter :-(

> 
>>
>> Something else comes in my mind. Would it good to document all this 
>> what we are talking bout somewhere on a webpage?
>>
>> Thomas
> 
> Perhaps so. Also a good idea to try to collect some people who have 
> similar interests and are trying to work on similar problems. I have 
> been trying to do that for a while, but with mixed and limited success.

May be, some people read this and have the same felling. Let's see what 
happens.

Thomas