Region render takes the same time as a full render...

[justavisitor]

It's counter intuitive, but to get the same quality for the full scene as the region you need to render it up to 100 samples. Looks like you could even lower the number of samples in the region render, try 8 samples or even lower. At 10 samples the noise in the full scene is really bad, but not so bad in the region render. Don't think samples per pixel, think image quality.

So you suggest that we change sample rates each time we switch between full (preview) Render and Region Render?

I guess that could work, though it's a bit awkward. So thank you for your help.

But the Region to Full render ratio still doesn't make sense:

I lowered samples to one in the new region render screenshot here (the one below) - and it still takes 2.07 seconds to render, while a full render of an image that is at least 25 times larger (and that's just the hair area) - in approx. the same quality - can be done in 9.35 seconds.

It doesn't add up: If you can render a full image in 5 minutes, it will still take 1 minute to render a 2% region.

And you still can't see your last full render simultaneously as you can in the video...

[grimm]

Yep, the ratio will never really works out because, like I said in my first post, the ratio of easy to render pixels are different than the hard to render pixels between regions and the full scene. In standalone I use the render region to clean up a particularly hard render area of my scene. In this way I can clean up a caustic or deep indirect lighted area and concentrate the full GPU power on it. You can't use the Blender plugin to do that yet, unless I'm missing something?

For traditional biased engines you would use the render region to quickly see what a particular area of your scene would look like. With unbiased engines it's not as much of an issue or needed I think.

[justavisitor]

Yep, the ratio will never really works out because, like I said in my first post, the ratio of easy to render pixels are different than the hard to render pixels between regions and the full scene. In standalone I use the render region to clean up a particularly hard render area of my scene. In this way I can clean up a caustic or deep indirect lighted area and concentrate the full GPU power on it. You can't use the Blender plugin to do that yet, unless I'm missing something?

For traditional biased engines you would use the render region to quickly see what a particular area of your scene would look like. With unbiased engines it's not as much of an issue or needed I think.

Thank you again Grimm! It's still a bit hard to see how it can be used in real life, but I get it now.

I rendered another example here though, and it took about half the time that it takes to render the full image (in an acceptable quality). If only it were possible to tell Octane to stop the render at a much earlier point. But this is @ one sample...

[grimm]

I think that if you are working on a very large scene, and you want to use the render region to check on a part of the scene, you could set the number of samples low. This shouldn't be too much of a problem as there are two sample settings, preview and final render. I set my preview samples low anyway to get better performance. I would very much like to have the ability to use render regions in the final render.

[justavisitor]

I would very much like to have the ability to use render regions in the final render.

I just tried - and you can do that. You just don't see the red border when you select the area.

But again, I don't know if the result is worth it: I usually work at 20 samples, which takes about 40 seconds to render. So I set it to 5 samples instead, and it took 38 seconds to render about 10% of the image...

[justavisitor]

OK, here's my conclusion:

I think I understand the reasons for the limitations of the feature now, thanks to Grimm.

But I've tried all combinations of samples and region sizes that I can think of - and I haven't found the feature to be useful at all in any 1GB+ scenes. Not even when samples are set as low as 1.

I haven't really experimented with smaller scenes so maybe it works in those cases. But then again, you really don't need the feature in a smaller scene...

[grimm]

I would very much like to have the ability to use render regions in the final render.

I just tried - and you can do that. You just don't see the red border when you select the area.

But again, I don't know if the result is worth it: I usually work at 20 samples, which takes about 40 seconds to render. So I set it to 5 samples instead, and it took 38 seconds to render about 10% of the image...

Excellent! That is one thing I just don't like about Blender, is it's reliance on keyboard shortcuts. If I don't see a button or menu with the option, I tend to miss the feature.

I wish I knew why the render region is like this, it's not intuitive at all. I suspect it's because of how CUDA works or how the Octane kernel logic works. The extra work that is done in the render region is not counted for some reason?

[aoktar]

Hey you guys are mistaken to compare values. There is a confusing logic on maxsamples. Maxsamples on render region mode, it doesn't reflect same calculation level as full render. I have added some estimation formula in C4D plugin to keep same level.

There is a estimated ratio like that:
Region_maxsamples = maxsamples * ( region_pixel_count / image_pixel_count)

Region_maxsamples value will be less than maxsamples. And render time will be less about this ratio. Please note that this may not guarantee the exact noise/calculation level for each area of image.

[grimm]

Interesting, thanks Aoktar!

[Synthercat]

It doesn't add up: If you can render a full image in 5 minutes, it will still take 1 minute to render a 2% region.

Remember that when rendering in blender you first need time for the actual model transfer to GPU RAM then the render starts so if the render is small in samples then most of the time will be spend for RAM transfering