Texture emitters use-case

[matej]

Until now I mainly used HDRI to light my scenes, but recently I tried to setup a "studio environment" using texture emitters. I'm puzzled how to properly use (only) emitters to light your scene. Basically the problem is that no matter the settings, you need to extremely overexposure the emitting surfaces to get any decent lighting on the subject.

So, I try to lit an object with an emitter representing a softbox. The emitter is a plane with texture emitting surface. The texture is applied to diffuse color of the material and efficiency or texture of the emitter node. The settings are:

The imager is set to linear with gamma & exposure at 1.0. The environment lighting is a texture with intensity 0.0 - ie. no env light. I used pathtracing with 32 samples.

The original power of the emitter (1.0) gives a proper visual appearance of a softbox, but to get any illumination on the object you need to use 20x, 50x, 100x more power which completely burns the emitter appearance (and thus its reflection). And even at 20x the power the object is still very dimly lit, almost invisible. The scale relation of the emitter and the object is something like trying to illuminate a person torso in a studio.

Is this behavior realistic? Am I missing something? It's possible to illuminate objects without completely over-exposure the emitters?

Take for example the situation of a room where the only light emitting surface is a TV. In reality the room would get quite some light from the tv screen, without the screen being burned and overexposed. I don't know how could be possible to achieve such illumination in Octane? And I mean not by faking it somehow, but using one emitting surface with a texture for tv screen?

[matej]

I find this light distribution really strange. The emitter and everything in the immediate vicinity is burned, but the object (which is at emitter width away from it), is almost not lit:

(the object's material is just a diffuse + glossy mix brightly colored, nothing unusual)

[bepeg4d]

hi matej,
are you on 257 or 258c? roeland has made some changing in the last version.
in 257 you must augment the sampling value in order to some effect from the emitter, in the last version this parameter doesn't seems to work
interesting argument, maybe you can do some comparison tests between the two versions
ciao beppe

[matej]

Hi, Beppe.

I use 258c. As far as I can tell the sampling rate param. doesn't do much. If I set it to the min value (0.001) it produces a bit more light.

I think this "ratio" how much the texture emitters look burned vs how much light they produce, has been present all the time. To produce usable illumination you need to raise the power so that the emitter is completely burned or scale it disproportionately big.

With burned emitters you get pure white rectangular reflections if your object reflects (ex: eyes of a person). As I mentioned it's not possible to realistically illuminate a room with a TV screen, for example (if anyone has such examples, please share) and still have a recognizable texture on the screen. A while back I also tried to do some torches, which I couldn't get right because the illumination was too dim unless I completely overexposed the emitter (to the point it didn't look like fire anymore, just a patch of pure white).

I don't have much technical knowledge, but something about how much light (and how far) is distributed from the emitter, doesn't seem right. Unless I'm really missing something.

EDIT:
I compared renders between 257 and 258 with the same sampling rate (lowest possible), and they look the same.

EDIT2:
With sampling rate to the lowest possible the brightness of the emitter and amount of light emitted looks a bit more in line. It wold be possible to set this parameter even lower? Especially since anything from 1.0 to 10.000 doesn't make much difference, the most useful values lie from 0.001 to 1.0

[roeland]

The sample rate parameter controls how often the different light sources are sampled. This is useful if the light from a light source has much more noise than the other light. It will not change the brightness of the lightsources. In the new version setting those to 1.0 should work fine.

If you set the camera response to linear, you should set the gamma value to match your monitor gamma (usually 2.2).

--
Roeland

[bepeg4d]

hi matej, i have made a simple test with an open box, some geometry and a plane with a texture emitter; the environment is set to 0, the emitter at 1 watt.
all seems to work as expected, probably because my texture is similar to a tv screen; your situation is a bit different, also in real photography if you exposed to see the soft light all around is black and viceversa.
the only solution for you, i think, is to create two planes, one with the soft light and the other with an emitter with opacity 0 and high wattage
ciao beppe

[matej]

Thank you Beppe & Roeland for your time. Your test Beppe, does indeed look right.

There were two reasons for my issue:

1.) Using the wrong gamma: I used 1.0 which I thought was right for the "linear" tonemaping representation, should have used 2.2.

2.) There seem to be something wrong with the images I used for my test. See the comparison between the "studio umbrella" image, which gives no illumination of the scene and a random image snatched from the net. The second version gives a right illumination vs emitter brightness ratio.

All the other settings are the same in both cases. Has anyone an idea why that umbrella image behaves so differently? It only gives good illumination if you raise power to the point it becomes overexposed. It's a .png file which I got by tonemapping the .exr version of it, with Luminance HDR. If I use the .exr version for the emitter it gives just slightly better illumination, but still far from the second test. Tweaking image gamma doesn't help. Could such difference come from a bit more bright areas of the second image?

Here is the complete scene (it will ask you for textures which are zipped alongside)

tex-emit-pak.zip

(760.56 KiB) Downloaded 148 times

[manalokos]

If you increase the gamma value instead of the exposure level you probably will get a less contrasty image but it will show illumination on your objects without overexposing the light source.

[matej]

Increasing the gamma together with exposure can help a bit, but too much gives lousy contrast, washed colors and more visible noise.

There something strange with the images I use
I re-tested it, this time with the softbox image (which is from the same pack the umbrella is) and it still gives almost no illumination, even if this time the image has more bright areas than the one with the formula car. Again all the emitter & tonemaping settings are the same in all three tests.

In the image node, there are three info parameters; size in pixels, size in bytes and a number. What does the number mean? In my case the formula car has 2, the others have 3.

[roeland]

It's not the images. For some reason Octane is always sampling the image texture at (0, 0) for calculating the illumination. This should get fixed in the next release.

--
Roeland