Cineversity

Hello,
This issue has been giving me a lot of trouble. I am trying to composite a 3D object and a Plane Object (as the ground) with a backplate.

I have a Sky Object lighting my scene with an HDR image. I have a compositing tag on my Plane Object and ‘Compositing Background’ is checked. The material was generated by the Camera Calibrator, by the way.

My issue is that when I try to composite the floor plane, the pass is very saturated and it renders the plane as white (see image attached).

I’ve also tried changing the blend mode to Multiply and matting it with an object buffer, but the result is, again, very saturated. I want some saturation, but only close to the object.

How should I set up my ground Plane material and render settings so the GI composites properly in the C4D-generated .aec compositing file?

Thank you!!!!!!

Hi Blastframe,

I have done a complete series here on Cineversity about Blend-Modes, if you like to dive into them.

In short, the Add mode summarize the values that you have in the background and foreground image. In this way you get the pixels values from both added. There is only one result possible, brighter than the anything in the scene, except for absolute black areas. Anything else—again brighter.

The simplest way would be darken (blend-mode) the top layer. Simple means here that you get not the right saturation or color in the shadows. As you have here a dark street, you might not be worried about that too much. All these blend modes work here only to a certain degree, which means, not really my suggestion in most cases; if we talk about photorealistic approaches. For Motion Graphics, in an artificial environment, that is a complete different story and much (again) simpler.

Multiply leaves everything that is black in the foreground as it is in the background, but any light in the foreground layer will “alter” the result.

If you work in integer (not float) some people like to use a mid gray as base “color”, and then use just Overlay in the compositing. I think for anything photorealistic we should use a linear workflow with practical footage from a calibrated and profiled camera, as well any image used in C4D only from profiled and calibrated cameras as well. BTW, to save an image in a floating point format does NOT linearize it. If the creator of the image has done adjustments to it, it might be ruined anyway. Many texture and HDRI supplier “fix” their stuff with some adjustments. A reason why I always suggest to use a MacBeth card. I discuss this all in my next tutorial series about texture, hdri and such.

I use the shadow pass normally as mask in an adjustment layer. With that I can control the shadow much more sophisticated. This allows to take the existent shadows in the background image as reference and adjust brightness as well as saturation/color, perhaps even the contrast, separately. If you measure the existing (background shadow) with the eyedropper, you get the RGB value for this, and a good idea how to adjust your shadow accordingly.

There are other ways to prepare the scene in C4D already, by using the practical footage, but you asked for an compositing fix, but I thought it is worth mentioning.

All the best

Sassi

Hi Sassi,
Thank you for your response.

I do understand Blend Modes pretty well, but as I mentioned above, using Multiply mode makes my image very purple.
Multiply on Backplate

I want the HDRI Sky object to give my Floor plane shadows and light bounce by my 3D object, but I do not want it to change the color of the entire backplate. Maybe I can mask it by rendering a version that has 0% GI Saturation?

Hi Blastframe,

Multiply in integer will never brighten your image, as the max value is 1.0. In that way multiply is not an option to add light. I have no idea about your setting nor if you use always the same profiles. If the HDRI for the GI has a blue/lilac tint, then this is the answer. Render a neutral gray object with that sky. Measure the RGB values after that. If you use an HDRI that was captured directly on the same set, then the background image and the HDRI do not match in your case (but it should if done precisely). If you have taken just “any” HDRI then you might have your answer. HDRI photography needs to be done carefully, even 10 minutes later on the same set, just with some cloud movements, the color temperature of the light can go off. A typical mistake is to set daylight to a fixed Kelvin value, and use that the whole day. Certainly not my advice at all.

If you are under time pressure, using your material, setting some curves to separate the shadow and the light information from it might help.

Purple images, can have another source, typically based on bracketing images. Which are initially developed in ProPhoto as color space, then with an HDRI algorithm that ignores the given profile (happens more than often), and is then later on wrongly interpret as sRGB or Adobe RGB1998. I do not see that in you image, but then again, I look at an (?) jpeg perhaps, which is certainly not my preferred image format for anything that is color sensible. Find the source of the color shift. I do not advice to just color-correct it, but that is more than often the way it is done, based on missing knowledge or most-likely a deadlines.

As I mentioned above, some people suggest to use a mid gray as base (floor in your case), and then use Overlay (which is a mixture of Screen and Multiply) works in both directions. Since screen-mode in its classic form will not work in float, I recommend against this. Well, the color shift will not go away from that, find the source.
Having said that, you do not supply if you work in integer or float at all., or any details of your color space workflow.

In R14 you have a new option, please check the Help Content>“Compositing Background for HDR Maps”. (New In R14)

All the best

Sassi

Thank you, Sassi.

You’re welcome Blastframe, good luck with the project.

As I mentioned in the series about light, any HDRI (360º/180º) is only valid for the point (literally) and time it was shot from. Any object that moves from this specific position (and time) is in danger to get a different light situation than it would have gotten normally.

The use of such HDRI sources seems to be understood as if even a larger scene could be illuminated with such, or any position change wouldn’t matter at all. I suggest to shoot always an HDRI from the position the C4D object will be placed in the scene. To use any HDRI that has no connection to the background (practical) footage, in “space and time” is of course not an advised practice at all and never will be.