Medallion with scratches (feedback required)

SATtva_ · Post by **SATtva_** » Thu Jan 22, 2009 5:19 am

Guys, I'm stuck and in need for feedback. I'm experimenting currently on reconstructing the photograph image of the silver medallion about 8 cm tall with the golden medal. The main problem I'm faced with is tiny scratches on the object surface.

If you look on the medal on the first image, it looks like has a quite high exponent, reflecting black studio. Scratches, on the other hand, are low exponent scattering light over their surface. Intuitively scratches are hollows thus need a bump map. But if you look on the third image, rendering medal with bump scratches gives in the result the opposite: scratches are darker than the smooth surface surrounding them.

Then I thought that maybe I could achieve satisfactory result with an exponent map, giving scratches surfaces lower exponent value. You may see the result on the fourth image (and corresponding material settings on the second one): surface looks uniformly smooth.

Is there something I missing here? What could you suggest or what would you do yourself to arrive at the desired result?

By the way, IOR of gold and silver are both a fraction of one, but Indigo does not allows IOR below 1. Is it ok? What IOR should be used then?

Kram1032 · Post by **Kram1032** » Thu Jan 22, 2009 5:36 am

As you exactly know which materials these are, simply use au.nk for gold and ag.nk for silver

For your exponent map: try to use C = 100 instead of 0...

Also, most idealy (right after super fine displacement which also simulates the roughness of the surface of a scratch), you might want to use both bump and exponent map...

why does "bump" have a title.png (whatever that means) while "exponent" has a bump.png?

SATtva_ · Post by **SATtva_** » Thu Jan 22, 2009 6:08 am

As you exactly know which materials these are, simply use au.nk for gold and ag.nk for silver

As you can see on the Blendigo screen, that's what I do. Nks provide IOR as well as spectrum? Great, didn't knew that!

For your exponent map: try to use C = 100 instead of 0...

It is already, as shown on the screenshot.

why does "bump" have a title.png (whatever that means) while "exponent" has a bump.png?

Artifacts of the WIP. ^__^ Title.png is the text on the medal, and bump.png is scratches texture which on the current stage is identical for both bump map and exponent map.

Kram, thank you for the input. I'll tweak settings a little end report back. If anyone else would like to comment, I'd be glad to hear.

Kram1032 · Post by **Kram1032** » Thu Jan 22, 2009 6:46 am

nk = "complex IoR"
they define an IoR N=n+ki, where n is the refractive index and k is the absorption at a certain wavelenght.

Oh, then I read in the wrong line

^^
Indeed... hmm.... try -250 b then? xD just a random idea^^

SATtva_ · Post by **SATtva_** » Fri Jan 23, 2009 6:20 am

Kram, thank you for the theory, I understand now.

Indeed... hmm.... try -250 b then? xD just a random idea^^

Negative values isn't possible in Blendigo. I haven't tried editing materials' xml by hand though (and honestly I don't remember what values are permitted by the spec[ification]).

I've played with material settings and textures for a while. I changed bump to 0.07mm (Blendigo rounds that up in the UI, so don't be surprised) and made texture 4096x4096px. Now it looks a little better but still scratches are too artificial like left with the nail. In the next step I'll recreate texture and make scratches with the sharpest brush possible.

Is there something additional worth trying?

Zom-B · Post by **Zom-B** » Fri Jan 23, 2009 6:40 am

raising the C for your exponent map doesn't make any sense (sorry kram).
By this you raise your maximal lowest exponent controlled by the exponent map to 200, so your exponent is always between 200 (for pure black on Exponent map) and 700 (for pure white).
a 200 exponent is imo still way to shiny for the scratched parts :/
I would go for a max of 300 for your medal...

Your ultra high resolution + very thin scrathes result in a very very small area in the rendering where the exponent map controls anything... its in a sub pixel area...

I would try to set more work into the exponent map... at first try for the scratches a pure black, and also blur this layer a little bit, so the gold gets a little less exponent near the scratches!
Also try something like a dirt map for your exponent map, something like you can find here:

http://www1.cs.columbia.edu/CAVE/databa ... _glass.php

Oh, and before I forget please please change the font you use for the cosmo stuff

CTZn · Post by **CTZn** » Fri Jan 23, 2009 7:35 am

also if one proportionates the dark of the text on the medal with the rather dark grey of the scratches, those have to be pretty deep, wile they don't look like so in the original pic. Make the scratchs of a much lighter gray; I guess their real depth is sub-millimetric.

SATtva_ · Post by **SATtva_** » Sat Jan 24, 2009 3:27 am

CTZn, a little more attention please: there are two distinct bump textures blended together: first with the text and second with scratches.

ZomB, many thanks for your advices. But what's wrong with this font? O_o

Well, I found the root of the fruitlessness of my experiments: Blendigo 1.1.14 exports uniform phong exponent despite exponent map set, and Indigo 1.1.15 makes precedence of uniform exponent over the exponent map. So in any case only uniform exponent is used, and one needs to modify material xml by hand. There was effectively no exponent map in all of my previous renderings.

I'm continuing my experiments. I'll report back in a couple of days.

neepneep · Post by **neepneep** » Sat Jan 24, 2009 3:49 am

Use of comic sans is a crime against humanity and is punishable by instant defenestration.

cookieofdoom · Post by **cookieofdoom** » Sat Jan 24, 2009 4:07 am

neepneep wrote:Use of comic sans is a crime against humanity and is punishable by instant defenestration.

+1

I think you might have too many scratches in your texture. The real scratches are fewer and farther apart, and more varied in severity. Also: I think your exponent overall is a bit too low. You can make out the shape of a lighting fixture in the original, whereas in the render it's just a blurry light impression.

Everything in the render is a bit sharper. The model could use some smoothing (either subsurface or some kind of modifier?) to make it more like the original, and the shadow is much more harsh than in the original.

Honestly, your most recent render is very good; better than I could do. I'm just comparing it to the original

Oh, one more thing... most of the scratches in the original image are up/down... yours are a bit more random....

Sorry if none of that is helpful.

SATtva_ · Post by **SATtva_** » Sat Jan 24, 2009 4:32 am

neepneep, yikes! I swear to replace it until the final rendering! ^___^

cookieofdoom, I suppose photographer relied heavily on post-processing: there are no reflection on medallion edges (I had to change material on the edges to the black diffuse to reflect that), and the shadow is unrealistically blurred which isn't possible with any type and size of light source or its position -- I tried.

Oh, one more thing... most of the scratches in the original image are up/down... yours are a bit more random....

Sure. Currently I'm trying to catch the "feel" of the scratches. When all parameters are optimal I'll prepare the final texture looking more like the original.

CTZn · Post by **CTZn** » Sat Jan 24, 2009 6:01 am

SATtva_ wrote:CTZn, a little more attention please: there are two distinct bump textures blended together: first with the text and second with scratches.

Aye Sorry !

Reciprocally you could have heard my notice to their important relative darkness with text, ie they are waaay too dark (deep), in my opinion, over.

SATtva_ · Post by **SATtva_** » Sat Jan 24, 2009 6:21 am

CTZn, you're right. I think I could fix that with the proper exponent map (now when I know how to make it work again

). And, yes, maybe 1mm is too way deep for letters.

Once again, thanks for your comments.

CTZn · Post by **CTZn** » Sat Jan 24, 2009 7:41 am

yep !

Make the scratchs of a much lighter gray; I guess their real depth is sub-millimetric.

*cough* scratches

Kram1032 · Post by **Kram1032** » Sat Jan 24, 2009 11:05 am

well, nice progress

keep it up!

Medallion with scratches (feedback required)

Medallion with scratches (feedback required)

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