Bluing / browning steel

[KilamMalik]

Hi,

I'm searching for a material that looks like blued steel. This is done by diving hot metal into oil. You get a surface which looks black, but has some blue reflections in it. I need it for the rendering of a hydraulic cylinders. Most pictures I found on the web are weapons, but maybe it helps:

http://waffen-blog.net/wp-content/galle ... eniert.jpg

The one on the bottom:
http://www.newphillysports.com/milsurp/1907.jpg

I tried some NK materials and mixed with color, but did not get where I want. Can anyone help me here?

That article on wikipedia tells, that the surface is then Fe3O4. Maybe that helps in selecting a NK?

http://en.wikipedia.org/wiki/Bluing_(steel)

Thanks,
Kilam.

[Zom-B]

yes, there are some related NK file, check out here: http://www.astro.uni-jena.de/Laboratory ... ul.html#B1

Poorly atm Indigo doesn't support "complex IORs"

Ono, any plans about changing this in the future???


There should be also some possibility to convert such a nk to a suitable one, or maybe even to a spectrum for usage with Specular reflection Phong??!

[CTZn]

Fe3O4 is rust

well I guess it's not, dunno. Some ferroxyde, do'h.

[KilamMalik]

@ZomB: Thanks. That looks exactly to what I'm looking for. Is there any documentation about NK or how I could convert that to an Indigo NK? I tried to use the dat file from that site as NK, but it gives me the error you are talking about. I don't know how the file is built up, it looks like a table, do I just have to remove the column that is there for the ocmplex IORs?

@CTZn: Fe2O3 is rust, Fe3O4 is the black oxide of iron.

[Zom-B]

I actually have no idea

maybe the fresh uploaded NK preview gallery can help you finding a "similar" material: www.zom-b.de/indigo/nk

The also exists the RGBTONK, a NK file converter coded by Tinman.
get it here: http://www.renderlight.co.uk/files/rgbtonk.rar

[KilamMalik]

Danke

The rgbtonk sadly crashes when importing the NK with the complex IORs. I will have a look into the NK gallery, Thanks.

[Zom-B]

Ok, on some other NK file I found some explenation:

Code: Select all

Column 1: wavelength in micron
Column 2: n 
Column 3: k

delete the first row, maybe by some excel style table transformation with open office or what ever...

[Kram1032]

counverting it to a tabulated spectrum might work like that:

search for the lowest wavelenght, use it as min
search for the higherst as max
find out the step and plug it in as step value
write n values for IoR
do the same for color with k values

No idea if that works though....

[KilamMalik]

I tried to compare the existing NK files with the NK file that I had downloaded. The differences:

The downloaded NK files first column is the wavelength. That seems to be missing in all the NK files if indigo.

There is some kind of header in the first row in the indigo NK files. It consist of 4 numbers, but they are different from NK to NK.

Now I tried to insert one first line into the NK I have downloaded. I just copied the values 1, 2, 4.5 and 100 from another NK. It now rendered, but it looked like copper.

Now I removed the first column of the downloaded NK with the wavelength. Hey, it is black with some blueish touch Its a bit too purple... maybe something else must be changed.

But I still don't understand what the NK parameters mean? Maybe the first row holds the interpolation values from the wavelength? The downloaded NK files had wavelength from 0.1 to 1000, so I try 0.1, 1, 10 and 1000 in the first row. Hmm, error again with the complex IORs. Or are this the steps like Kram1032 said?

I removed the lines with wavelengths above 100 then to see if it is interpolated... no, seems not, exactly the same result.

Ok, changing the first line to 1, 1.5, 2, 45... now it is green... hmmmm. No clue.

Now only doing small changes... but for the first two values the color changes rapidly. Seems I had luck in copying just that line from another NK Getting closer now, changing the 4.5 to 5 and then 5.5 makes it more black with only a little purple touch... I got it


Maybe somebody could post a documentation on the NK files

How can I attach pictures here, is it only possible to link? I would like to put my NK and sample pic in here.

[KilamMalik]

Can't attach NK extension... copy and paste:

Code: Select all

1 2 5.5 100
0.96397	0.53633
0.98179	0.53994
0.99873	0.54191
1.01456	0.54244
1.02909	0.54172
1.04217	0.53998
1.05366	0.53744
1.06347	0.53436
1.07154	0.531
1.07786	0.5276
1.08243	0.52439
1.08528	0.52162
1.08644	0.51954
1.086	0.51833
1.08404	0.51825
1.08065	0.5195
1.07595	0.52229
1.07009	0.52686
1.06324	0.53339
1.05557	0.54215
1.04735	0.55333
1.03883	0.56716
1.03038	0.58384
1.02238	0.60356
1.01533	0.62642
1.00977	0.65249
1.00631	0.68173
1.00563	0.71396
1.00838	0.74885
1.01522	0.78587
1.02674	0.82432
1.04333	0.86329
1.0652	0.90173
1.09222	0.93848
1.12392	0.97243
1.15947	1.00265
1.19776	1.02854
1.2376	1.04994
1.27795	1.06715
1.31808	1.08073
1.35761	1.09129
1.39649	1.09935
1.43477	1.10517
1.47252	1.10882
1.50963	1.11025
1.54582	1.10943
1.58076	1.10644
1.61404	1.1015
64529	1.09503
1.6743	1.08756
1.70105	1.07976
1.72571	1.07226
1.74873	1.06564
1.77073	1.06029
1.79248	1.05639
1.81482	1.05377
1.83851	1.05195
1.86408	1.0501
1.89172	1.04718
1.92113	1.04203
1.95149	1.03373
1.9816	1.02167
2.01005	1.00586
2.03548	0.98686
2.0569	0.96582
2.07382	0.94412
2.08632	0.92325
2.09501	0.90459
2.10093	0.88915
2.10544	0.8776
2.11012	0.87008
2.11655	0.86616
2.12618	0.86476
2.13999	0.8642
2.15819	0.86234
2.18001	0.85703
2.20367	0.84659
2.22675	0.83039
2.24687	0.8089
2.2622	0.78362
2.27185	0.75645
2.27577	0.7293
2.27454	0.70366
2.26906	0.68056
2.2603	0.66057
2.24915	0.64394
2.23636	0.63067
2.22252	0.62065
2.20812	0.61371
2.19351	0.60963
2.17895	0.60822
2.16465	0.60929
2.15078	0.61267
2.13747	0.61819
2.12481	0.62574
2.11293	0.63518
2.10187	0.64642
2.09173	0.65933
2.08259	0.67384
2.0745	0.68984
2.06755	0.70726
2.06178	0.72601
2.05728	0.74598
2.05409	0.76708
2.05227	0.78923
2.05187	0.81232
2.05292	0.83626
2.0555	0.86093
2.05959	0.88623
2.06524	0.91204
2.07246	0.93826
2.08125	0.96478
2.09161	0.99149
2.10352	1.01828
2.11698	1.04505
2.13194	1.0717
2.14838	1.09812
2.16625	1.12424
2.18552	1.14995
2.20611	1.17519
2.22799	1.19987
2.25108	1.22393
2.27532	1.24729
2.30065	1.26992
2.327	1.29174
2.35429	1.31273
2.38246	1.33282
2.41142	1.352
2.44112	1.37024
2.47147	1.38751
2.5024	1.40378
2.53385	1.41906
2.56574	1.43333
2.59798	1.44659
2.63053	1.45883
2.6633	1.47007
2.69624	1.4803
2.72928	1.48954
2.76236	1.49783
2.79541	1.50515
2.82838	1.51155
2.86123	1.51704
2.89388	1.52166
2.92631	1.52543
2.95844	1.52839
2.99025	1.53057
3.0217	1.53201
3.05274	1.53275
3.08334	1.53281
3.11347	1.53226
3.14309	1.53112
3.17219	1.52944
3.20074	1.52726
3.22871	1.52462
3.25609	1.52157
3.28286	1.51815
3.30902	1.51439
3.33454	1.51034
3.35942	1.50606
3.38365	1.50156
3.40723	1.4969
3.43015	1.49211
3.45242	1.48724
3.47403	1.48231
3.49499	1.47737
3.5153	1.47246
3.53496	1.46761
3.55397	1.46283
3.57236	1.45819
3.59011	1.45371
3.60725	1.44941
3.62379	1.44533
3.6397	1.44149
3.65505	1.43793
3.6698	1.43466
3.68398	1.43171
3.6976	1.42912
3.71067	1.4269
3.7232	1.42508
3.7352	1.42368
3.74668	1.42273
3.75764	1.42222
3.7681	1.42222
3.77808	1.4227
3.78756	1.42373
3.79656	1.42529
3.8051	1.42742
3.81316	1.43014
3.82075	1.43346
3.82788	1.43742
3.83456	1.44202
3.84077	1.44728
3.84651	1.45325
3.85179	1.45993
3.85659	1.46735
3.86091	1.47554
3.86472	1.48453
3.86803	1.49434
3.8708	1.50502
3.87301	1.5166
3.87464	1.52911
3.87562	1.54265
3.87595	1.55724
3.87554	1.57292
3.87434	1.5898
3.87227	1.60796
3.86924	1.62752
3.86511	1.64862
3.85974	1.6714
3.85295	1.6961
3.84453	1.72296
3.83422	1.75231
3.82169	1.78456
3.80645	1.82044
3.78784	1.8609
3.7654	1.90685
3.73799	1.96037
3.70431	2.02463
3.66355	2.10291
3.61317	2.20612
3.55437	2.34344
3.49292	2.55011
3.45905	2.86675
3.57613	3.36073
4.2297	3.89584
5.29536	3.65486
5.58089	2.75204
5.27338	2.30164
4.99574	2.15206
4.79451	2.12371
4.64853	2.14606
4.53811	2.19315
4.45057	2.25561
4.37571	2.32914
4.30873	2.41376
4.2464	2.51111
4.18673	2.62427
4.12896	2.75799
4.074	2.91908
4.02563	3.11709
3.99284	3.36465
3.99349	3.67697
4.06259	4.07194
4.30193	4.56237
4.82917	5.04819
5.67047	5.26281
6.55652	5.01649
7.18903	4.32132
7.22563	3.58335
7.03966	3.13676
6.8177	2.85791
6.60578	2.70413
6.44044	2.64547
6.30933	2.62433
6.20065	2.62472
6.11927	2.64522
6.05657	2.67582
6.00391	2.71152
5.9678	2.75412
5.93886	2.79913
5.91716	2.84658
5.90429	2.89654
5.8939	2.9472
5.89215	3.00008
5.89164	3.05326
5.89707	3.10792
5.90397	3.16295
5.91543	3.21914
5.92805	3.27565
5.94471	3.33326
5.96184	3.39102
5.98309	3.45004
6.00433	3.50906
6.02923	3.56934
6.05465	3.6298
6.08212	3.69106
6.11142	3.75303
6.14088	3.81507
6.17388	3.87867
6.20688	3.94226
6.24137	4.00657
6.27798	4.07193
6.31458	4.13728
6.35338	4.2038
6.39358	4.27106
6.43378	4.33833
6.47621	4.4069
6.52007	4.47629
6.56392	4.54568
6.60927	4.616
6.6569	4.68775
6.70454	4.75949
6.75218	4.83124
6.80349	4.90541
6.85514	4.97981
6.90678	5.05421
6.95902	5.12902
7.01497	5.20642
7.07092	5.28382
7.12687	5.36122
7.18282	5.43862

[SATtva_]

KilamMalik, to attach a picture use "Post Reply" button. It's not possible in "Quick Reply" form.

[KilamMalik]

Thanks, SATtva, so here is the screenshot.

It uses the fe3o4.nk from the link that ZomB provided above. I changed it a bit to make it work in Indigo.

[CTZn]

Good job !

So you just removed the first column and tweaked color with the 4 first numbers on the same line ?

Since you used Fe3O4, was the color right in first instance ?

It would be cool if Indigo could natively extract tabulated data from those nk files :/

Interesting topic !

ps: the exponent of the reference seems higher

[KilamMalik]

I think, I had a bit luck. I compared the NK that I downloaded with the NKs from Indigo. So I have seen, that the Indigo NKs had 2 columns where the downloaded one had three columns. But the Indigo NKs had that first line containing 4 numbers that were missing in the downloaded NK. So I just took the first line of one of the Indigo NKs and inserted it into the downloaded one after I have removed the first column.

Now, using 1 2 4 100 for the first line gave me a nearly black part with some purple, but a bit too much. So I played around. Changing the first number changed the color completely (looked like copper when I changed it to 1.5). Changing the second had also a gread impact, the third one had only little impact and the last number nearly no impact. By changing the third number up to 5.5 I got what I wanted.

So to your question: The color was nearly right with my first attempt, but I think it was luck. Using the first row from another NK from Indigo gave completely wrong results. But it now looks like I know that blued metal looks like.

My assumptions

The "real" NK files have the wavelengths in the first column. As that is removed with the Indigo NKs, I assume, that the first row gives the hint for the wavelength. Maybe the first and last number is the minimum and maximum wavelength. The numbers two and three are maybe steps? Not sure.

There is a credits.txt in the NK folder from Indigo. It contains a link, but the page does no longer exist. The site is from some optical research company. Would be interesting, if their NK files are Indigo or standard format and who converted them and of course how?

If I would know the exact format, I could create a converter, so lets continue this thread to find out the details... somebody must know it

[KilamMalik]

Ok, found a description of the format:

http://people.csail.mit.edu/jaffer/Free ... ell_2.html