kilgota@xxxxxxxxxxxxxxxxxxxxxx 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
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