Hi,
Is it possible to mix in the same project, clips with different resolutions, that then get rendered WITH THEIR OWN associated resolution, untouched... at the very least, not downgraded.
The issue chiefly arises with pictures that sometimes replace for a few seconds or so, the video. I cannot choose a global resolution enough that the picture I put here and there aren't reduced, for it would mean 16k res and the project would be +100 Go
I would like... rendering option for separate clips, not the whole project.

Is something like that possible ? I know the format .mkv can do that, but can kdenlive use that ?

Hmm, never heard about mkv being able to hold videos of different resolutions *at the same time*. But who knows ...

kdenlive (and, I believe, and other video editing software) can work with source material of different formats and resolutions. Meaning, you can mix 4K video and HD video and JPG and PNG, etc. But it can work only with one set resolution for the current project. That's why you have to specify that at the beginning for a new project (yes, you can change it at any time even while still editing but again only for the entire project). That means you cannot have one part of the project render at 4k, the other at HD, and the next at UHD (almost 4k), and so on. The project settings also determine, IIRC, the default output size although you can change that in the render dialog window in order to, say, render a quick preview of your project in 720p or even less for speed reasons.
So the source material is always "adjusted" to fit the project resolution settings. Which means that low resolution source material would look pixelated in a high-res project. Likewise, when zooming into lower resolution images or video clips it loses sharpness/crispness faster and looks pixelated even at lower zoom factors.
The advice is always try to get your source material in the highest resolution you can afford or manage. Of course, the higher the resolution, the bigger the file size.

Hope this helps ...

so you're saying that it's not possible, if I want one frame (his res picture) to stay still for a time, to allow it to keep it's max res ?

I've never heard of a video with different resolutions. Are you really sure that possible? If so, you could try rendering in sections and putting them together with ffmpeg

I'm no pro so I may have misunderstood.
In any case, that it's not possible more simple (for damn still pictures is stupid.
if I can put stuff together with ffmpeg then it is possible after all, so why can't software cater for that ?
I could do what you say but that would give +15 sections, to gather manually, it's ridiculous... I just want good res still picture without pixelizating the whole video...

https://ieeexplore.ieee.org/document/8361567/
A mixed resolution based high efficiency video codec (HEVC)
So it IS possible. Viva 2022, a simple concept a complete noob can come up with, and that would improve quality and reduce work (I suppose no pixelization) somewhat exists but isn't common.

Merging/splicing videos together with ffmpeg still requires the sources to be of the same dimensions. You either upscale to the highest resolution, or downscale to the lowest (recommended) or scale all to whatever target resolution you need. I am no expert either but my online research has yet to produce conclusive evidence that it is possible to have multiple sections of a video with different resolutions in the same container (i.e. mkv, mp4, etc.).

Secondly, the referenced paper describes a new codec to be used with stereoscopic videos in which frames (except for I-frames) are down-sampled and stored at a lower resolution than the source material. When played back (decoded) it up-samples it to its original resolution. The method exploits optimizations of human eyesight (the brain doesn't process all the signals coming from the eyes thus coping better with the data onslaught but sacrificing detail). I believe that the described approach can be transferred to the way videos are processed, stored and replayed currently. I would like to see that, though

You need to distinguish between resolution of your source material, the project resolution and the resolution of the rendered product. They are different things yet related. Even if you render videos and stills independently at whatever the optimum resolution is individually, joining them will require a resolution to be defined to which the source material will be converted in the process.

Most commonly, 1080p is used for playback. So you would set your project resolution to something like that: 1920x1080. Your source material can be any resolution, any format and with any aspect ratio. kdenlive will handle almost anything for editing and playback in the project monitor but will adjust the source material to fit the project resolution. For example, if you drop an image of 78x50 pixels into the timeline of a project with the above setting that image will be scaled up by more than 2,000%. Not good.

The better the resolution of your source material the better the perceived quality of your final render. Avoid using source material that needs to be scaled up by a lot in order to fit the project/render resolution. If you need to use low-res source material try to do something with other than scaling. For example, put a video wall together with the same picture (needs copies on several tracks with scale down and moves to different positions), or put something else around it like a frame.

I'll take time to stomach that. For I got very, very little of it
ok, it really isn't possible at all, in short, run your video, than show a high res picture with as much pixel from the source as it can, to fit into the window's size, and go on with the rest.
I understood globally "no"... ok

you can create a 4k project... and create every format during rendering using the "rescale" option on rendering dialog. (but ... i have no idea about details quality.. and on how "rescale" works)

Wait, won't that multiply the video's size 50-fold ??