EXR - Store Image as Tiles = No Colour Depth

[pokoy]

Let's hope it's possible to fix. Considering how contrast can easily produce negative values the limitation may be by design.

You're right, the tone mapping controls are very effective. Still, I would love to have a way to save both at the same time, a tone-mapped version and another one without any tone mapping applied.

[atelieryork]

Let's hope it's possible to fix. Considering how contrast can easily produce negative values the limitation may be by design.

You're right, the tone mapping controls are very effective. Still, I would love to have a way to save both at the same time, a tone-mapped version and another one without any tone mapping applied.

Definitely.

There is already a way to save both - use the Save VFB button in render setup dialog. It will save a "raw" version of the VFB with no tone mapping applied. So you can save that, then save out from the VFB as normal for your tone mapped version.

[Nekrobul]

Use these settings

[Juraj]

Indeed don't use the contrast, works strangely in my opinion, not sure what kind of curve it produces but seem to work mostly on blacks. Needs filmic :- )

[atelieryork]

Use these settings

I will try these on Monday but I'm pretty sure that these won't fix the problem.

Are you positive that you're not getting the issue if you use Contrast >1.0?

[atelieryork]

No dice on those settings. Same issue.

Is anyone from Legion able to acknowledge this issue? I'd like to rule out user error (aka I'm being stupid somehow). I feel like someone somewhere would have spotted this at some point before me...

Cheers,

[Ondra]

this is the expected behavior

look at this color curve:


When you have no tone mapping, there is straight line between input and output. If you extrapolate that into overbrights (colors with intensity > 1.f, or > 255), you still get the same straight line - see red in the image.

But if you use contrast, you use a curve similar to the one in the image. Now imagine you extend that into overbrights. Where will the curve go? It is depicted in blue. It shows that all overbrights are compressed into very shallow range of colors (which quickly degenerates into single color because of numerical precision limits).

This is not a bug and it makes sense for 2 reasons:
1) if you used contrast, you are working in non-linear space, so it is no longer safe to change exposure of the image. Same as using gamma, highlight compression, etc. You can always do tone mapping in post-production and use the correct linear output from Corona.
2) by using the contrast function you explicitly requested for the near-black and near-white range to be destroyed to increase the differences between black and white. This is what the contrast function does by definition. If there is some other implementation that is not as aggressive, I could switch it, but the function we use currently was the best one I was able to find based on a number of desirable properties.

[atelieryork]

Hi Ondra,

Thank you for the detailed response! I think I now understand what this all means and why it occurs and I had suspected it was something like this causing it. The bottom line is that using Contrast in the VFB or CameraMod destroys your ability to re-expose the image, and this is a very serious limitation that basically makes the tool useless to anyone who might want to re-expose in post (which must be a lot of people, I would guess). I very, very frequently render my main image which overall has a very nice exposure, but some areas might be undesirably over exposed, so I simply reduce the exposure in post and comp it back in where needed. A typical use is to get rid of those nasty aliased overbright areas around light sources. They disappear when underexposed and you can get all that for "free" without having to re-render the whole image again with a lower exposure, when using 32-bit with retained dynamic range.

The main benefit of Contrast, for me, is that I was really enjoying the workflow of getting the image looking basically 10/10 in the VFB using Contrast - for example when producing nice, bright white interiors with dark black floors and crisp highlights - it's very difficult to achieve that look without considerable post-production but corona's Contrast tool basically solved it, so that's a shame it's not usable without this issue.

I really don't know how you can solve it but I would strongly suggest that you put up a warning message in the console when the user has Contrast > 1.0 so at least they know that this will kill all dynamic range in their image once rendered. 32-bit EXR (half or full) becomes pointless once Contrast is used, so it's also a matter of disk space and RAM usage in post (I bet there are people using Contrast >1.0 who are rendering animations to 32-bit EXR for no reason - you might as well render to 8-bit PNG/TIF/TGA etc.) and save a huge amount of space and RAM).

I hope you can find a workaround or alternative implementation that retains at least some dynamic range. Contrast is a very nice tool indeed and helps to establish mood very quickly, so it's a shame to lose it.

[Juraj]

32-bit EXR for no reason - you might as well render to 8-bit PNG/TIF/TGA etc.

Rendering to linear output seemingly yes,...but color-depth is always welcome even for clamped images. And since .exr is such effective format (disk size, channels,etc..) it makes absolute sense to use it no matter of linear/non-linear post-production phase.

[Ondra]

yep, you can still do a shitton of stuff in LDR image that keeping the 32bit resolution per channel makes sense