Re: libv4l: possible problem found in PAC7302 JPEG decoding

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

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