On 02/01/2010 10:23 PM, Németh Márton wrote:
Hello Hans,
while I was dealing with Labtec Webcam 2200 and with gspca_pac7302 driver I recognised the
following behaviour. The stream received from the webcam is splitted by the gspca_pac7302
subdriver when the byte sequence 0xff, 0xff, 0x00, 0xff, 0x96 is found (pac_find_sof()).
Before transmitting the data to the userspace a JPEG header is added (pac_start_frame())
and the footer after the bytes 0xff, 0xd9 are removed.
The data buffer which arrives to userspace looks like as follows (maybe not every detail is exact):
1. JPEG header
2. Some bytes of image data (near to 1024 bytes)
3. The byte sequence 0xff, 0xff, 0xff, 0x01 followed by 1024 bytes of data.
This marker sequence and data repeats a couple of time. Exactly how much
depends on the image content.
4. The byte sequence 0xff, 0xff, 0xff, 0x02 followed by 512 bytes of data.
This marker sequence and data also repeats a couple of time.
5. The byte sequence 0xff, 0xff, 0xff, 0x00 followed by a variable amount of
image data bytes.
6. The End of Image (EOI) marker 0xff, 0xd9.
Now what can be wrong with the libv4l? In libv4lconvert/tinyjpeg.c, line 315 there is a
huge macro which tries to remove the 0xff, 0xff, 0xff, xx byte sequence from the received
image. This fails, however, if the image contains 0xff bytes just before the 0xff, 0xff,
0xff, xx sequence because one byte from the image data (the first 0xff) is removed, then
the three 0xff bytes from the marker is also removed. The xx (which really belongs to the
marker) is left in the image data instead of the original 0xff byte.
Based on my experiments this problem sometimes causes corrupted image decoding or that the
JPEG image cannot be decoded at all.
Hello Németh
I remember the problem as I was working on the PAC7311.
http://www.kaiser-linux.li/index.php?title=PAC7311
This is the code I used in the JPEG decoder to remove the 0xff 0xff 0xff
0xnn markers.
See http://www.kaiser-linux.li/files/PAC7311/gspcav1-PAC7311-20070425.tar.gz
decoder/gspcadecoder.c pac7311_decode()
Thomas
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