What's the best way to find the first JPEG frame tag (0xFFD8) in a proprietary video format?

I have a proprietary video file which is essentially a bunch of JPEG frames, plus a 1B-byte frame header on each frame, and an arbitrary-length file header.

After I find the first frame, I can read the entire file, and have already made my own MATLAB viewer for this format.

THE PROBLEM: the very first frame can occur anywhere in, say, the first 5kB. I need to find it quickly; JPEG files begin with a 'FFD8' tag, but this tag can be at an even or odd byte, so simply reading the first 5k once as uint16s won't necessarily find it.

MY SOLUTIONS: The first was to read the first 5k as uint8s with a zero offset, and then to do the same with a one offset; then compare the zero offset with 255, which is hex FF; compare the one offset with 216, which is D8, and AND them:

first_5k_zero_offset    = fread( hd_fid , 5000 , 'uint8' )  ;
                          fseek( hd_fid , 1 , 'bof' )       ;
first_5k_one_offset     = fread( hd_fid , 5000 , 'uint8' )  ;

find( and( ( first_1k_zero_offset == 255 ) ,                ...
           ( first_1k_one_offset  == 216 ) ) )

This works, but I've had to do it a few times and I feel like I'm missing an obvious better way. I also did the equivalent approach by reading zero- and one-offset uint16s, and comparing them to 65496, which is hex 'FFD8.'

I snipped a short portion of a video, just a few frames, and attached it in case you'd like to try playing with the file. So how about it? Is there a smarter way?

Thanks!

Answer 1

[Walter Roberson]

Read it once as uint8. strfind() [255 216] in the resulting vector.

Or

find(Vector(1:end-1) == 255 & Vector(2:end) == 216, 1, 'first')

[Brad Clymer]

Thanks! I've gotten too buried for a speed comparison, but I'll try one when I have time and let you know. Most appreciated!