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 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
