Re: libv4l: possible problem found in PAC7302 JPEG decoding

["Németh Márton" <nm127@freemail.hu>]

Hi Thomas,
Thomas Kaiser wrote:
> 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()

Do you remember whether this code was working properly always?

Regards,

	Márton Németh