Optimisations for the feeble computer

[colinleroy]

Hi,

and thanks for this very small JPEG decoder. I based my Apple II Quicktake 200 decoder on it thanks to its very small memory requirements.

I have hacked it A LOT (including ripping out every JPEG subformat that the Quicktake 200 does not produce, and hand-rewriting core functions into assembly), but I feel these few changes could benefit anybody.

I hope you'll like them :)

• Huffman decoding:
  • we never use mMinCode's high byte, and we subtract mMinCode's low byte from mValPtr unconditionally, so do it when building the tables and drop it entirely.
  • use a dedicated mGetMore bool instead of a special-case mMaxCode 0xFFFF to consume one more byte
  • decodeNextMCU: branch a bit less selecting the Huff tables
• General:
  • getExtendOffset: use arrays instead of functions and switches
  • skipVariableMarker: jump to the desired place much faster than left-shifting 8 times every byte we want to skip

Here is a before/after callgrind run (before on the left, after on the right):

Full callgrind logs (gcc -g -O0):
callgrinds.zip

[richgel999]

Thanks! I'm not working on this project at the moment, but I appreciate you posting this.

[colinleroy]

Thanks! I'm not working on this project at the moment, but I appreciate you posting this.

No worry! I'll keep pushing some changes I use on my own decoder as long as they change 0 bits in the output images :)

Long shot question (as I think you understand the jpeg format better than me): in my decoder, I decode to grayscale because it's going to be displayed in 256x192 1bit anyway, so I skip the MCU coeff computation and the IDCT for that 3rd component. However, I still have to "pretend", consuming bits for nothing. Like you do for reduce mode, but with a if (mcublock < 2) condition instead, and I also throw out the DC component there.

I presume there are no markers in jpeg files that would tell one where in the file to skip to. That would be 2400*3 markers for a very edge use case. And if they existed, you'd have used them.

Anyway, thought I'd ask :)

[richgel999]

I presume there are no markers in jpeg files that would tell one where in the file to skip to.
Not really - the best you can get are restart markers, but they aren't guaranteed to even be in the input file.