Odd Opacity Results

dlm4001

I have been working a chiffon fabric (material)  for a dress I created. While working on the texturing,  I noticed an odd opacity issue.  I only need a hint of opacity cutout for this fabric but I increased it for the sample image below to make the effect more obvious.  Notice that the opacity behaves differently over the model's skin.   On the sides you can see the backside of the dress through the front side of the dress and it has a fairly normal sheer material effect.  Then notice the fabric over her thigh and knee does not really appear trasparent.  Her skin seems to either show up white beneath the fabric, or the opacity is not working over that region. There is no skin diffuse color coming through.  If I make the fabric almost completly see through (Opacity cutout below 30%), then I start to see skin diffuse color coming through, but this is not useful for this project.   

This may be related to the light emitter through a transparency map issue (IRAY) but I am not sure.

Does anyone have any information of how to reduce or eliminate this unwated effect?  Workarounds?  lighting changes?   Having seen may IRAY renders for architecture including sheer curtains that looked simply amazing, I find it hard to believe that IRAY can't handle this.  Anyone have any ideas?

 

 

Gordon228

first i have seen this but then agen i do not set up lights. the only think i can suggest is to remove all lights and then see. as you have stated it might have something to do with the lighting. (wish i can check this but can't atm)

agent unawares

Try adding translucency.

Gordon228

thats not the issue here the issue is that there is but when you get to the character model it's as if there is none.

dlm4001

I have experimented with this problem and discovered a few things I though I woudl share. The two hints provided above were instrumental, thanks for both.  While I still haven't found the balance I was looking for at this camera angle on this dress, I am getting closer.

I replaced the front spot and rim spot with emissive surfaces (planes) and adjusted for the same light level.  I got the same 'harsh' trasition between transparency where the model's thigh was behind the dress.  This leads me to believe that it is not an issue with directional lights and opacity within IRAY.  

I added some translucency to the material and this helped.  However in order to get the diffuse color of the thigh showing through', I had to add too much and the dress looked more like a nightgown.  (too trasparent.)  I think that a small amout of translucency mixed with Opacitiy cutout is a good combination for the material I am trying to simulate, but still, this harsh trasition. 

By playing with the lights I determined that 'effect' it is a combination of things. 1.  The rim light is backlighting the dress and causing a silhouette of the model's thigh on the front side of the dress. 2. A very white spotlight is illuminating the material of the dress and on the light colored material, appears to illuminate the material more over the silhouette *shadow of the model's thigh cast on the front of the dress.  (I discovered this by playing with the temperature of the front spot.)  

I am closing in on the right mix.  Turning down the lumens on the rim light and tighting the beam angle so less of the light falls on the hem area of the dress helped.  Loweing the temperature and lumens of the front spot helped.   Adding a cross angle light with a low temperature to bring out the skin tones a bit in the hem area.,  Increasing the enviroment lighting to make up for the lower lumens spots helped.  And adding a touch of translucency to the surface helped.    Once I have a good mix of these items, I will post and result image as a follow up.  I have not run into this problem before so any further discussion is appreciated.