On Thu, 5 Mar 2009, Thomas Kaiser wrote:
Hello Theodore
kilgota@xxxxxxxxxxxxxxxxxxxxxx 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."
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.
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.
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.
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.
Theodore Kilgore
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