On Fri, 16 Jan 2009, Olivier Lorin wrote:
<snip>
Post gamma or white balance correction seemed to be more part of the webcam
capabilities than the image state so that the new API to get these features
is quite logical.
Hmmm.
While one is in the process of planning ahead, it it good to think of
everything. You have not yet seen any code for the sq905c cameras in
webcam mode, yet. Well, actually, you can if you want to. The needed
sequence of operations is used in the support for
"gphoto2 --capture-preview"
in libgphoto2/camlibs/digigr8/library.c.
What happens with this camera is, the frame data has been compressed and
therefore it is needed to know how much data is present for a frame before
downloading the rest of the frame. The length of the frame header is set
at 0x50 bytes, so one downloads that first. Here is a sample of one such
frame header:
0000 ff 00 ff 00 ff 00 ff 00-ff 00 ff 00 ff 00 ff 00 ................
0010 ff 00 ff 00 ff 00 ff 00-ff 00 ff 00 ff 00 ff 00 ................
0020 ff 00 ff 00 ff 00 ff 00-ff 00 ff 00 ff 00 ff 00 ................
0030 ff 00 ff 00 ff 00 ff 00-ff 00 ff 00 ff 00 ff 00 ................
0040 e0 5a 00 00 3b 3e 50 3b-36 00 ff 00 ff 00 ff 00 .Z..;>P;6.......
Now, most of this is just filler. The data size for the frame is to be
found in bytes 0x40 and 0x41 and it says that 0x5ae0 bytes must be
downloaded for this frame. I have never seen the next two bytes with
anything in them, so my reasonable conjecture is that they are similarly
used, as part of the data size field. It is just that in practice one
never sees a frame with so much data in it.
Now, my comment here is directed toward bytes 0x44 through 0x48. I do not
claim to know exactly what they mean. However, I would strongly suspect
that they have something to do with contrast, color balance, intensity,
and so on. Really, logic would seem to indicate that there is no other
point to providing those entries, which certainly do vary from one frame
to another and the readings in them do seem in some vague sense to have
something to do with the local light conditions, and sucn.
Therefore, confronted with my experience with these cameras, I would tend
to suspect that
"Post gamma or white balance correction seemed to be more part of the
webcam capabilities than the image state"
may not always be the case. For, here, with this camera, it appears that
some kind of related information is being passed with each individual
frame. It is also more certainly the case that the camera has no other way
to pass such information. In particular, there is no long initialization
sequence. One just turns on the stream.
Theodore Kilgore
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