Compressed JPEG as more colours than RAW

[Phil4000n]

I made some tests with my Canon EOS T1i/500D shooting in RAW+JPEG

What I wanted is to reproduce a same size JPEG as the one create with the camera to compare the "qualitity" as well as test shrinking the images to a smaller amount of pixel.

Test reveals:
1/ Shrinking from the RAW to a 1920x1080 pix is lighter in Kb than if JPEG is the source. How comes???

2/ Different softwares produce different compression results at same parameters.

3/ If I use the canon Digital Photo Professional software (3.6.0.0) I can't get the same JPEG result as with the camera.

4/ Using other software than canon produces lighter Kb files and more faithfull to the RAW (maybe noice reduction algorithms on 12800 ISO image). As an illustration I have a 21Mb RAW image, the JPEG produces with my Canon is 6916Kb, and from a soft I choose Q=100 I get 6373Kb.
Comments?

 

[Garbz]

1) Two theories. Either your software handles the compression differently between RAW and JPEG (i.e. mimicking the blocksize and subsampling of the original JPEG vs using a software default for the RAW), or the data in the JPEG is in someway trashed in a non-visible way which would account for a difference in the efficiency of being able to encode it).

2) They are not the same parameters. If you click export in GIMP you will get a more complete list of JPEG compression parameters including optimization, smoothing, subsample selection, DCT storage formats, along with actual hard coded specific variations in things like the quantisation matrix used to interpret the JPEG blocks. JPEG is not a simple compression algorithm, there's a heck of a lot that goes into it and it's all highly customisable while still being compatible with a range of decoders.

3) Bizarre. If there's one program that I expect would have resulted in the same it's DPP. Typically RAW converters will apply their own interpretation of what data should look like. I would have expected that from Lightroom or UFRAW, but I would have expected that Canon's DPP could faithfully reproduce the camera JPEG from the RAW.

4) See 2)

 

[djacobox372]

Jpeg is an EXTREMELY complex compression format, you aren't going to make any sense out of it.

It doesn't surprise me that a jpeg made from a jpeg would be larger then a "1st generation" jpeg, as the smoother and cleaner an image is the better it will compress with jpeg, but the compression process itself will degrade an image slightly making it less smooth and less clean.

 

[pgriz]

@ Garbz: I've been told that the JPEG algorithms in DPP are not the same as encoded in the firmware. I've tested this (RAW+JPG in the same shot), and couldn't often reproduce the JPG result out-of-camera when working on the RAW. A techie who does testing for Canon has confirmed that Canon regards their implementation of the JPG algorithm as proprietary, and therefore will not divulge the exact steps they take in firmware.

 

[Garbz]

@ Garbz: I've been told that the JPEG algorithms in DPP are not the same as encoded in the firmware. I've tested this (RAW+JPG in the same shot), and couldn't often reproduce the JPG result out-of-camera when working on the RAW. A techie who does testing for Canon has confirmed that Canon regards their implementation of the JPG algorithm as proprietary, and therefore will not divulge the exact steps they take in firmware.

This ultimately has nothing to do with the JPEG algorithm and everything to do with interpreting RAW data. Still that's a lame excuse at best. DPP is a Canon product. They should provide the option of recovering the image exactly as the camera would render it in their own RAW converter.

 

[pgriz]

DPP is a Canon product. They should provide the option of recovering the image exactly as the camera would render it in their own RAW converter.

I agree. But apparently they don't. I can't see what they would reveal that would make their competitive situation less, but perhaps they are tweaking with firmware what they don't want to call attention to in hardware. Speculation on my part, along with annoyance when I get a nice JPG rendering out-of-camera and I can't reproduce it with DPP or Adobe.