Yes, I confess that I still do not really fully understand why there are two sliders in Kdenlive's new render dialog. Or how they may interact or not. I've just checked the Kdenlive documentation just to be sure that it doesn't already explain my question, and it doesn't.

What I think I may understand is that there is a slider for resulting file size versus quality -- if I wnt better detail then compression will be not as good as when I opt for smaller file size. I would imagine that this slider setting applies to individual frame compression only ... or does it also apply to the temporal compression along a series of frames in time?

What I don't really understand yet: why another slider for quality versus speed? I've read another recent comment by Vincent but I'm uncler whether it does hint at this: do this also influence the picture quality, thus also affecting compression? If yes, in which direction? What does speed here refer to, the rendering speed? So if I'm opting for speed, will compression be worse, resulting in larger files? Or does the picture quality go down mainly without affecting file size? If I go for quality, will rendering take more time? Then what about file size, will it increase too with quality? Or will quality result in both better compression, yet without sacrificing picture detail?

Hello TheDiveO,

I have to answer because I am the only one to understand my logic, even JB didn't follow me on that topic
You can have a look at the profiles.xml to see the real parameters it affects in MLT (~ffmpeg):

Quality slider (vs size) corresponds to CRF (H26x/VPx) or QScale (MPEG2...) since all profiles are now VBR, you can see the parameter actual value when you check "More options". So setting better quality will just increase the file size (normally with a log/exp link : +6dB quality leads to size approx *2).
If you have custom CBR profiles, then the quality slider & video spinbox will control the bitrate.

Then I added a new tuning parameter : encoder speed vs compression efficiency.
Codecs have options that can make compression sensibly more efficient, but at the cost of longer and longer encoding time (eg motion estimation ranges etc -- have ever seen x264 ultrafast to veryslow/placebo presets?).
This means increasing speed with VBR quality constraining, you get a constant quality with larger files ; in the CBR case it is constant file size with poorer quality.
This is one of the main criticism against 0.9.10 : switching to MLT presets, it changed from fast defaults to very slow ones (which made many people unhappy, and did bring the focus on the multithread option removal that doesn't play that much).
Moreover, here I had different presets for quick project preview & final (long) compact export for diffusion (web etc).
I estimated that having different presets makes the list long and confusing, so I made the option tunable.
But having it shown by default reveals to be confusing as well, so I recently moved it to the "more options" pane, and tried to change a bit the labels & tooltips.
Needs more brainstorming + documentation!

Cheers,

Vincent

Vincent, thank you very much for taking your time to explain. This helps a lot.I don't see a problem with the two sliders as such, I only found the slider labels too terse.

So one slider is "video quality versus rendered file size".

And the other slider is "rendering speed versus file size".

I know that each simplification has the seed of incorrectness in it, but do you think these two short classifications catch the gist correctly?

with git commit 7a1c2fcdc04f 4March2016 the encoder speed slider it greyed out for me in the rendering dialog. Why I wonder. Is it only valid for certain profiles? Which ones ?