View Full Version : bad/weird aliasing (float related)

05 May 2009, 04:34 AM
Hi, everyone,

I have to bring this topic up again because I still have not solved this problem for this particular scene. Related threads here regarding bad aliasing of float renders are read and treatments proposed are tried. But the issue detailed below is IMO somewhat different.

In my renders, I get some bad aliasing highlight around when over-bright pixels meet dark ones; however, I also get bad aliasing (like dot-stripe) where pixel values are not of big different. Explained in pics below: (all images are rendered as EXR and re-saved as JPG in Photoshop)

Here is the zoom-in. Highlights around the left and the lower area of the car engine grill are weird looking where the pixel values are all smaller than 1, but interestingly the most noticeable highlight at the top has better AA where pixel values are over 5. (I sample value in Nuke)

I tried to use higher AA sampling value like 1,3 with contrast down to 0.06. The dotted area looks a bit smoother but still not satisfied. Shown below:

I prefer to preserve the over-bright value to the post so I can apply some bloom or flare effect. But with those not-over-bright dot-stripe, I have no clue how to "utilize" those "dots"?
I attach the exact scene that can re-create those dotted-looking highlights. Please take a look and advise me how to solve this issue.

Down the mb file here (

05 May 2009, 01:23 PM
You could try to bring down the highlights to values like 1.5-2 with the simple tonemapper. Then sampling should work a bit better.

05 May 2009, 01:46 PM
I meant mia_exposure_simpl. Set "Knee" to .9 or 1.0 and raise the compression and check if that helps.

05 May 2009, 01:47 PM
Hi, brainspoon, about the highlight value of the tonemapper you mentioned, were you refer to any specific parameter of one of the mental ray's lens shader (like mia_exposure_simple or mia_exposure_photographic)?
The weird dotted looking highlights appear around where pixel values are not of big difference. I don't know if a tonemapper will help in this particular situation.

edit: I added a mia_exposure_simple and tweaked the "knee", but the dots are still there. :(
The geometry should be pretty smooth as I checked already. What else could cause this weird looking? This is different from the wavy looking aliasing that I got before when "hot" pixels meet "cold" pixels and for that I can actually "utilize" the super-hot pixels in post.

You could try to bring down the highlights to values like 1.5-2 with the simple tonemapper. Then sampling should work a bit better.

05 May 2009, 07:48 PM
try setting up passes for your scene and then see if the bad sampled areas are happening in the specular buffer, the reflection buffer, or both

when using physical lights with super bright values, i've found that the specular highlight is always going to be so bright as to be useless (and often with artifacting)

you could also try setting the hl_vs_refl_balance of your material to zero and see what happens

if it's the specular that's causing it, you'll just have to fake highlights in your reflection channel or something. you can throw cards in front of your lights for this... use a ramp texture with a high value for its color, like 3.0-5.0

i know it's a hack but i don't know how else to handle this problem, especially when rendering to passes (since the buffers don't get tonemapped)

el diablo
05 May 2009, 08:39 PM
is this specular, reflection, or specular reflection....This info would be helpfull...el diablo

05 May 2009, 09:09 PM
Galakgorr: Actually, the problem is most obvious in my specular pass. I simplify the scene by taking out the HDR map, no raytracing, no FG and what not, leaving only part of the car geo and one directional light. The thing I don't get is what cause the dotted highlight even when values of connecting pixels are not of huge difference? (for certain camera angle, I do get bad-aliasing highlight and that is caused by mix of super bright and dark pixels. I can deal with that in post).

el diablo: Those weird highlight, IMO, is specular highlight or you can say specular reflection of the light source. I don't have reflection map in the scene and the camera BG color is completely black.

If downloading the scene and just hit render, one will know better if I describe the issue not clearly in context.

el diablo
05 May 2009, 09:21 PM
run a normal pass and see it the geo jumps too much in angle in that area...el diablo

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05 May 2009, 09:21 PM
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