Encode: do not perform rate control for single-tile lossless

[boxerab]

Currently, for single tile lossless, rate control is performed, although it is not needed.

This can lead to lossy encoding if there are code passes at the end of a code block that do not
contribute to a rate increase. The rate control algorithm will remove these code passes,
leading to potential lossy encode.

[boxerab]

Correct solution is to simply include all passes in a single layer for single-layer lossless

[rouault]

Do you have examples where that occurs ?

[boxerab]

No, I can't remember the exact case, but I saw this happening when I removed the limit of 10 resolution levels. Also, if you look at the logic in the code, you can see how this might happen.

But, even without a test image, there is no reason to perform rate control on single layer lossless image.
You just need a method to detect when this is the case, and skip the rate control.

[rouault]

Also, if you look at the logic in the code, you can see how this might happen.

Actually I don't see how that could happen. Lossless encoding implies cp->m_specific_param.m_enc.m_disto_alloc == 1 and tcd_tcp->rates[0] == 0. Which implies in opj_tcd_rateallocate() that goodthresh = min; which is the minimum of dd / dr for all passes of all code blocks (

openjpeg/src/lib/openjp2/tcd.c

Line 616 in a55c024

goodthresh = min;

). Then opj_tcd_makelayer() is called with this threshold and checks in

openjpeg/src/lib/openjp2/tcd.c

Lines 291 to 292 in a55c024

	if (thresh - (dd / dr) <
	DBL_EPSILON) { /* do not rely on float equality, check with DBL_EPSILON margin */

if (thresh - (dd / dr) < DBL_EPSILON), which should be always true. So all passes should be selected. Am I missing something ?

[boxerab]

hmmmm. Well, I guess that was a false alarm then :)

[boxerab]

How about lossless with multiple layers ? Could the final layer be missing some passes ? If so, that would be a problem.

[rouault]

The same logic will apply for multiple layers. For lossless encoding, the last layer should have tcd_tcp->rates[layno] == 0, in which case all remaining coding passes will be selected. I just committed a fix for opj_compress help which indicated 1 instead of 0 for this lossless terminating layer

[boxerab]

For -r flag, 1 indicates a final lossless layer, while for -q flag, 0 indicates a final lossless layer.

So, looks like everything is correct then. Closing.

[rouault]

For -r flag, 1 indicates a final lossless layer

Actually that doesn't match my experiments. You really need to set 0 to get all the remaining coding passes in the final layer. 1 gets you almost to lossless, but not completely

[boxerab]

Well, what I would do is keep the 1 for compression ratio flag, and then convert to 0 in the code. Compression ratio of 0 is a bit counter-intuitive. You could let the user know that 1 == final lossless layer.

[boxerab]

i.e. set rates[layer-1] to 0 if user has final ratio of 1.

[boxerab]

Also sanity check that ratios are decreasing and >= 1