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confusing problem i recently noticed


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hi. i noticed on a recent render (and in subsequent test scenes) that my trees appear to get very dark (very dark!) the further away they are from the camera.


1. the trees are using leaves which are a mentalray arch&design material with a tga file in diffuse and copy of the tga file in cutout, using an alpha channel in the tga file.


2. the leaf material is not set to backface cull, nor is the mesh


3. i'm using a mr sun/sky, FG is off, GI is off, skylight illumination is set to from IBL (using the mr physical sky)


4. even if i switch FG on, and switch between IBL/Non-IBL lighting, results are the same.


there is another tree on the left that looks OK from a distance as well as close up. if i put that material on the trees that are "dark" they render fine from a distance. likewise putting the "dark" material on the tree that renders OK makes it dark from a distance.


i've examined the properties of the bitmap etc... can't seem to figure it out. i've tried increasing the output of the diffuse map but when i render close up, it's way to bright.


all the trees in question are using "flat" leaf meshes generated from speedtree architect. they are edit meshes and are two triangles. i've checked normals and they are correct, i've looked at smoothing and no smoothing, no difference. it seems like it's a material issue but i can't figure out why?


the tga's were created in photoshop. they have an alpha channel and saved with that alpha channel. the diffuse is one bitmap and the cutout is another bitmap.


my only other thought was that it's just shadows darkening everything on that particular tree? oh and there is no reflectivity at all, it's matte.



edit: please forgive the ground material haha not working on that until i can figure out the dark tree thing :)




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got it!


like it shows, close up was fairly good/ok. it's the distance shots that gave me angst. normally i set the cutout map bitmap filtering to none because it is supposed to save rendering time. however, i left the filtering for the diffuse version of the map to Pyramidal. when i switched the diffuse map to no filtering, i'm no longer seeing black trees.


my tga files have black backgrounds. i tried with a different color background (pink) but it didn't make a difference unless Pyramidal filtering was on in the diffuse map... then i saw the pink.


so it seems distance from camera has a big deal to do with filtering of bitmaps.


to summarize, i turned bitmap filtering off for both diffuse and the cutout map.

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  • 2 weeks later...

I don't think it's specifically distance from camera so much as rendered size of texture. If it takes 20 pixels to get across a leaf then at the edge the color of a leaf may come from only two or three texels. If it takes half a pixel to get across a leaf then the color of the leaf comes from all texels. It would be cool then, I guess, if filtering obeyed opacity. Or we can just remember to give our images appropriate backgrounds.

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