Author Topic: Virtual ColorChecker  (Read 8118 times)

2015-03-30, 18:52:16

dubcat

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I tried to recreate my ColorChecker Passport today just for fun, to see if it would work.
If you haven't used one of these in real life before, eat3d has a very nice video about it here (Linear Workflow)

The main chart is using color values from the official manual, the other chart is using values from this document. The technical report document has some reflection % values as well, I don't really know where to use them, but I have tried to sync them with the Albedo RGB values. I'm not 100% sure about the glossiness values either, the color swatches are really rough on the real thing. I read somewhere that it's painted paper, so you can't stand in the rain with it.

Looks like imgur destroyed the images.

This was my first test


In my second test, I just used corona converter on a scene and pressed render. 16 passes


I went over to the couch and threw down the ColorChecker


Imported them into Lightroom. Linearized the settings, adjusted white balance and the main grayscale values in proof mode. You want to use the gray color to the right of white, under the green, on the upper chart for white balance. Or you can use the single white balance swatch.


Cross images


I have included a zip with a scene file (saved as max 2012) that has the 3 checkers, and a chart with the sRGB values if you want to play around with it. Shout out if you know where we can use the % reflection and what glossiness values this rough paper is using.

PS...
Someone is hiding bananas in the couch :0
« Last Edit: 2015-03-30, 19:30:54 by dubcat »
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2015-11-29, 23:22:47
Reply #1

Juraj Talcik

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Ressurecting thread older than 120, hope no one will frown too much ;- ).

I've been pondering over this myself at occasions, although never dived deeply.

Dubcat, how do you think "tonemapping" affects this whole thing ? I watched the Eat3D video myself but what he refers to as "linearizing" obviously isn't that or is it ? It's just "flattening" any camera response curve the .raw file have had. Does that actually create correct Albedo value to be used (would be super handy for scanning all the material swatches clients send me physically) ?

Also, with regard to tonemapping, at which point is the chart representative inside 3D scene ? At linear level without any tonemapping ? Or how does further highlights crushing affect this ?
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2015-11-30, 02:04:40
Reply #2

dubcat

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Quote
"It's just "flattening" any camera response curve the .raw file have had"
Yep

When the image is flat, you color correct the image with the "ColorChecker Camera Calibration" app, and then fix the values with Photoshop/Lightroom.
You end up with the real life albedo value that you can use as pbr reflectance in the high pass filter technique.
There can still be pixels that clip black or white in the picture, but that's because they are in shadow or overexposed. We only care about the proper exposed real life value of a material, we need cross polarization to get the "real" real life value ;)

It is known that every chart differ, and as time goes by you get shit and scratches on the swatches. I wouldn't trust my life on it if your client is strict about the color, but it is good enough to get values for albedos. Maybe you should try SilverFast, I have heard good stuff about that scanner / calibration software.

Photographers use the chart differently, they use it for white balance and to get the correct colors with "ColorChecker Camera Calibration". They don't need the real life values, and the 100% calibrated images look pretty washed out.
If I remember correctly I had some problems with Corona and the "ColorChecker Camera Calibration" app. Because Photosop/Lightroom is looking for the camera model in the EXIF Data. And there is no Canon/Nikon etc data in the Corona image. I tried to fake it with a EXIF Data editor but failed. I haven't tried again after this old test.

I would put my money on 32bit linear render, fix the values with CameraRAW and then tone map with ArionFX/Nuke/Fusion.
If I remember correctly this test was done with tone mapping, and I lost a lot of detail on the bright curtains and in the shadows.

Now that I think about it, what a weird test, so unorthodox. Is this technique really better then a 50% grey sphere in the digital world ? Maybe I should do a retest when I get time and include a 50% sphere!
« Last Edit: 2015-11-30, 02:17:58 by dubcat »
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2015-11-30, 02:38:27
Reply #3

Juraj Talcik

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Photographers use the chart differently

Which is why I am surprised this technique was suggested to me on quite few occasions from 3D guys, including Ondra himself :- ) Perhaps I should finally look into using it a bit

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2015-11-30, 03:00:24
Reply #4

dubcat

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You know what, now I'm curious. I'm going to do the tests straight away. RIP sleep

I only did 32 passes, didn't have time for more.

RAW Render and Tone mapped Render



So here I adjusted the White Balance in CameraRAW. The luminance swatches are correct straight from frame buffer. I just checked the values in frame buffer and adjuster exposure.



The tonemap test was kinda funny. When the tone mapped version is calibrated in CameraRAW, it looks like the original render only with shitty contrast.
Further down you will see that 3D LUT Creator can retain the contrast. But it still looks like the original render.



I used the built in ColorChecker correction tool in 3D LUT Creator.
Colors, white balance and values are corrected.



3D LUT Creator. Only adjusted values and white balance not colors.



3D LUT Creator. Tone mapped version. Only adjusted values and white balance not colors.
You can see that this gives better result than my quick CameraRAW fix and it is fool proof.



3D LUT Creator. Tone mapped version. Colors, white balance and values are corrected.



Conclusion: Tone mapping doesn't do shit if you are going to calibrate your image.
In this case tone mapping is like taking a big dump on your picture, and then you scrape it off with the color checker to reveal the original picture.
« Last Edit: 2015-11-30, 17:43:10 by dubcat »
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2015-11-30, 15:40:15
Reply #5

dubcat

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Bumping since I'm done with the initial test and only did edits in the post.

The reason why 3D LUT Creator is color shifting the image is because of my ColorChecker in the render. 3D LUT Creator is expecting other RGB values and I'm using the values from ColorChecker Passport.

If you want to try 3D LUT Creator, use this chart instead. This chart matches the 3D LUT Creator checker 100%.
I'm going to use this chart in future tests.

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2017-12-23, 10:29:34
Reply #6

arcmos

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By dubcat. Thank you for the virtual colorchecker! Do you have any news about the use of virtual colorcheckers in combination with 3D Lut Creator?

Maybe changes or updates of the rgb color swatches?