Discussion on using a photographers approach to Rendering (Grey cards, Color charts etc)

[CoastalVis]

I wanted to start a discussion around some of the more background approaches some of may you use to achieve a more photorealistic end result. As its quite time consuming and difficult to achieve all of this in 3d, the techniques you adopt in Post production can really help push and image to the next level. Saying this Ive been watching alot of Photography tutorials recently, where I have been trying to replicate/integrate some of techniques Photographers use in my 3d workflow. I am wondering what behind the scene techniques some you guys use, I was thinking of making a digital Color chart and or grey card/ball, chrome ball etc and rendering that in scene to aid in normalizing the render in post and getting the most out of your images. Do any of you use these kinds of tricks in your work at present? if you do it would be great if you could share some of your workflows that could maybe take the guess work out of post production/ 3d scene setup.

[nick_martin]

Im also very interested in this and have been thinking about doing the same thing. Jorgen (the guys who makes awesome HDRI's) has a bit of info on his facebook page - https://www.facebook.com/jorgenhdri/

[Ihno]

That's an interesting read:
https://forums.chaosgroup.com/forum/...-ibl-equipment

There were more very interesting threads featuring joconnell on similar topics. He really knows what he's talking about.

Like this (watch the video he posted):
https://forums.chaosgroup.com/forum/...-as-it-s-units

And this is also very interesting:
http://forums.cgsociety.org/showthre...9&page=1&pp=15

[CoastalVis]

Ive been experimenting with this in my sparetime recently which I dont have much of so bare with me. Just to check my workflow to bring a uncallibrated hdr lit image back into callibration.

1. Set scene up with hdr as main light source, add plane with digital version of Color Chart so its in camera and correctly exposed.
2. Render out as 32bit linear.
3. Convert linear image to srgb color space
4. Use software to compare the difference between rendered rgb values and actual original rgb values of color chart.
5.Create LUT

The problem im having creating the lut is because Real cameras create .nef/.dng images which are the only formats supported by Photographers calibration software(X rite. macbeth etc). Is there a software where we can using 32bit linear images to create a profile?

[Ihno]

See the links I've posted.
Its the other way around.
Vray gives you a perfectly linear image with real colors. (which is what photographers are after when they use color charts)
Its the camera which changes the colors of the reality for the flavor of a visually appealing image. Use a color checker to linearlize the camera image.
The inverse of the color corrections needed to do this will be your LUT profile for vray.

You could use the workflow from this video https://vimeo.com/107961975 (but erxpose your image right when you shoot the photo)
And export a LUT afterwards. Dont know if you can export an inverted LUT from lightroom. But you could invert it in fusion (free up to 4k) and export that as new lut.

[CoastalVis]

Hi Ihno,

thanks for your reply. I understand how the photographers do it but im trying to copy this process in 3d on uncalibrated hdris. I have several that are unbalanced. I thought it might be possible to reverse engineer the photographic approach by using a digital version of the color chart and rendering the chart out in 3d and using the render to reverse calibrate the hdri which would give me an lut to sue with hdri? Your referring to how to shoot hdri and textures? or am I getting confused?

[Ihno]

Ah yes I got you wrong, sorry.
I linearlized the brackets before merging them when I made hdri's. I think that would be the usual way.
The resulting image still needed some tweaking though.

You want to lineralize renders which are made in an uncorrected HDRI?
I may be wrong but I don't think that works this way. The light would still behave realistically inside of your scene. You'd still get a linear image.
Using an uncorrected hdri would be just as if your lighting had a different contrast or color. Might look less real, yes.
But I think in most cases some corrections to the hdri using fusion or something like that would do the trick.
At least for a full cg scene.
Vfx could get a little tricky.