Same Grass Problem

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Same Grass Problem
by Jason Stirret on Jul 27, 2012 at 11:31:18 pm

I am running into the same grass issue as before and still have no solution. Determined to find a solution for this I am posting again. The link for the project file is attached, any advise or suggestions are highly welcomed!

4441_grass0007projectforthecow.zip

A special thanks to anyone who can help with this.

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Re: Same Grass Problem
by Adam Trachtenberg on Jul 28, 2012 at 6:57:18 pm

I think the problem is that you simply don't have enough grass to hide the ground underneath when it rises up, reducing overlap. What you might try is increasing the hair count considerably, but then use a density map to place more of the grass where you need it (without making render times even crazier).

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Re: Same Grass Problem
by Jason Stirret on Jul 28, 2012 at 7:28:48 pm

Hi Adam, thanks for your help. After some testing I don't think that this is the problem. If I disable the plane which the grass sits on the hair renders fine (though the sky can be seen through it), i.e. the actual grass strands do not change color or hue midstream. I suppose I could just composite two images in Photoshop but then I would not know where the original problem was.

I have tried setting the plane to black with no specular to see what happens. And as I suspected it is not a dark line in the horizon but that same light line which discolors the grass. I have GI turned off on both sides, the plane as well as grass and that didn't work either; so I don't think GI is at the root of my problem. I will beef up the hair count density map it and try your suggestion.

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Re: Same Grass Problem
by Darby Edelen on Jul 29, 2012 at 11:17:31 pm

Can you upload an image of what you're expecting to see (with the plane disabled)? I'm not seeing a problem with the grass colors, so it'd be easier to identify the problem you're seeing with a second sample image that doesn't show the problem.

Darby Edelen

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Re: Same Grass Problem
by Jason Stirret on Jul 30, 2012 at 7:28:32 pm

Here are two pictures isolated so you can see the problem and what it should look like. Thanks for your time by the way. If I cant find a solution I'll just layer it in Photoshop using hard light. But the fact remains I want to know why its doing this!

Thanks Darby

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Re: Same Grass Problem
by Darby Edelen on Jul 30, 2012 at 10:29:37 pm

What we're seeing there are shadows cast from the ground geometry onto the grass.

The Shadow properties under the Illumination tab of the hair material can be adjusted to reduce this.

Shadow Density will affect all shadows, including self shadows, so that likely won't be the best option.

Reducing the Received Shadow will make the shadow the ground casts less intense, but it will also affect the shadows cast by any other objects so that might also not work.

Lowering the Back Shadow is probably your best option.

Darby Edelen

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Re: Same Grass Problem
by Adam Trachtenberg on Jul 31, 2012 at 5:28:13 am

I think you're on the right track. I'd focus on the back specular and back shadow options as it does seem like the problem is coming from the rim light.

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