Chaos Corona for 3ds Max > [Max] Resolved Bugs

DeadClowns Buglist

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DeadClown:
I will use this thread for further bugs I may find (if that's ok).

build 30.8 2012
- 3dsmax crash when map in emission slot
_Keymaster: FIXED
- 3dsmax crash with multitexture map in diffuse slot (which is a plugin but it would be awesome if it would be supported)
- corona light (rectangle mode) is only creatable in one direction (1st click and drag to several directions). In my opinion it would also be more practical if the light was created with flipped normal direction (currently its pointing towards the camera, so i have to rotate it about 180° everytime.
_Keymaster: FIXED
- 3dsmax crash in scatter when distribution scatter objects frequencies are set to 0
_Keymaster: FIXED
- I noticed a very noticeable change in the look of the images attached to this post. Especially the bottom part of the humming top looks very purple now. Left image is alpha v2, right image is build 30.8 2012

Ludvik Koutny:
The color change could be either moved whitepoint, changed the way blackbody color is computed, or changed colorspace from RGB to XYZ... Important question is which of the results is correct :) We will have to dig into this... :)

DeadClown:
I've prepared a little scene (no maps) which should be perfect to test those things. The scene is saved with alpha v2, the 30.8.2012 version is made by opening the scene and pressing the "production defaults" button ;)

Ondra:
Hi,
I'll look at the other bugs when I have the time (in 1-2 weeks).

The color change is in my opinion because the internal color space of Corona was changed from sRGB to XYZ. Because XYZ gamut covers the entire range of visible colors, it allows Corona to deliver more accurate results than other renderers in situations, where extremely vibrant emissive colors are created (blackbody emission <1700K, sun at sunset/sunrise, heavily colored refraction attenuation, sky models near horizon (even in the middle of day)). The result still needs to be clamped and converted to sRGB, but the internal calculations are in XYZ, so the clamping is only where absolutely necessary.

Secondary effect of XYZ is that its primary colors are imaginary - they do not represent any real colors, they cannot be reproduced or perceived. This means that you never encounter for example XYZ(1, 0, 0). Such types of RGB colors (e.g. RGB(1, 0, 0)) are common, and cause many problems during rendering. XYZ transforms 1 0 0 to 0.41 0.36 0.18, which is much better to compute with. I will demonstrate with examples:

Scene lit with neutral light: there is practically no difference between RGB and XYZ:


Now lets light the scene with purely green light. Since the right wall is RGB(1, 0, 0), and the light is RGB(0, 1, 0), its component-wise product is RGB(0, 0, 0), which is black. XYZ handles the situation much better, since there are no purely monochromatic colors after transformation:


Blue light: same story, this time both walls result in black when using RGB:


Blackbody radiator which emits mostly red, but also other colors: since the left wall is RGB(0, 1, 0), it cannot reflect anything other than green. Because of this, the wall remains green and does not take any tint.


Now, back to your scene: I am guessing, since you are using blue object color, and yellow lighting, which is complementary color to blue, that in the RGB the colors are not mixing, and lighting is not affecting the color of blue object. In XYZ, the blue object is tinted (as is everthing other in the scene), producing the purple.

Now, what is "correct"? The answer is both - because the term "blue" can mean a wide variety of very different spectrums. XYZ just uses one that is not as extreme, resulting in more plausible shading, but occasionaly less vibrant colors, when it is needed for the material to correspond to the rest of the scene.

DeadClown:
Thanks a lot for your explanation. I didn't know there was such a technique as xyz. It sounds convincing! I thought about this blue color - orange light mixing too but I'm accustomed to this rgb look from vray and other renderers, so it looked a bit weird on first view ;) I'm sure it's a good technique so I'll do some experiments with it.

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