A specific shadow color on a particular object can be done in the material room for the object that is showing the shadow. I have demonstrated this in the image above.

I use a Blender node to pick two colors for the ground. I drive it with the Diffuse node. The Diffuse node all by itself can detect where shadows fall. By inverting it with the subtract node, I get a high value where there is shadow. I can then adjust the Blending value to decide how much to change the color. Thus, I can choose a color for the shadow.

In this case, I made a pink ball and made the ground shadow pink as well. This is nice as it gives a tone to the ground making it seem like there is some light bouncing off the ball onto the ground, a really cheap form of cheating global illumination. It's not a general solution, but can be used for special effects quite well.

You don't have to just use a color for the shadow. You can use patterns, too. Here I used a Fractal_Sum node to create little spots of color in the shadow. It looks like the glittery ball is throwing pink specular reflections onto the ground.

You'll have to click to see the full size view in order to be able to see the effect clearly.

How about when shadow catcher is being used on ground? I have problems with shadow only being that one color. And I am using 1 IBL. When I use another light to make the shadow color, I get no shadow. I am trying to get my shadow color R:16 G:28 B:54

Great advice you got so far.
One thing one of my art teachers always emphasized is that realistic shadows are never solid black, and also that they always have some blue and opurple in them (when outdoors)
When indoors, they have a bit of dark part of ambient light coloring to them.

Quote - Here's another useful illusion using the material technique.

You don't have to just use a color for the shadow. You can use patterns, too. Here I used a Fractal_Sum node to create little spots of color in the shadow. It looks like the glittery ball is throwing pink specular reflections onto the ground.

You'll have to click to see the full size view in order to be able to see the effect clearly.

That is a nice illusion this one, great idea bagginsbill !!! quick and fast, simple is beautiful!

@ **Robo2010

**yes that is the problem here the shadows not always fit with the shadow catcher especialy when the orginal ground is reflective in the original photo so photoshop and layers are the last chance....until B.B come with a better idea ;)

Attached Link: My Gallery Image - Spanish Tourist Attraction

Heheh. I have done this before. Examine my linked image (warning - tasteful nudity).

In the notes there I said this:

*This was suprisingly difficult. Had to make a custom shadow catcher that gave a blue tint to the shadow to match the real shadows. The lighting was pretty easy though. I hacked together an IBL probe myself to get the general lighting and then one infinite light at 120% took care of the rest.

*I'm at home and my Poser file for that is at work. If I recall correctly, I did not use the "shadow catch only" option. Instead, I attached something similar to the above shadow detector (1 - Diffuse) into the Transparency input of the ground.  This makes the ground become more opaque where there is shadow, and transparent where there is light. Then I placed a dark blue color in the Ambient and turned Ambient_Value = 1, with zero Diffuse_Value and Specular_Value. By adjusting the amount of transparency and the exact color, I was able to match the shadow exactly. 

Basically this is blending a specific color with the background picture where there is shadow and no completely transparent where there is light.

When I get to work tomorrow I'll open that file and post the exact material I used.

That's cool idea too, thanks for sharing.. now what about the shadow catcher is there anything we can do about ? or is that a closed case

Quote - I would go one step farther than BB does and use the ambient to force the shadow color.

Yeah almost. You forgot the rest of the floor needs to be transparent for the superimposed image on a background, if you're talking about that scenario.

cross post!

Yes I do that too in my renders not using the shadow catcher feature at all , to put a shadow on a light colored ground is fine not big deal look only on my Vacation pic from my gallery , but the most problem is if the ground is to reflective as in the image by Robo2010**
**

 but the most problem is if the ground is to reflective as in the image by Robo2010**

Image (BG), to reflective (lighting and shadow)? 

Trying to grasp something here.. **

I mean the ground reflect in the Image BG the light from the sky very intensive , the shadows under the car are much more darker there is not light from the sky not from the sun  , the position of your model is in a lucky spot where the part of the shadow excist already but the final render have a double shadows now.. so the parts of the shadows under the model should be darker and the rest need to match the other shadows of the image that have a ::blue tint:: and with shadow catcher feature it is not possible to do that correct.
You should render this out as a layer without the background image, remove the shadows from the places where they are already in the BG image and tint the rest of the shadows to match it and perfect image is finish..

Attached Link: The other Forum (DAZ) :Smile:

This BG photo I took many years ago, months after the situation in Katrina, gas prices started sky rocketting day after. The pumps all over our city (Edmonton, Canada), were lined up of cars, for blocks. I was out taking photos capturing the moments. This was the only image I had on my Hard drive that had reasonable shadows to show, and give examples at the moment.

Attached Link: Gas Pump Line up.

But, here is the actual photograph if you want to play with it. 

And stuff to look at.

http://www3.telus.net/public/tfantini/

Thanks , the image give me some funny idea for later :)

Here's a demo render. I'll be posting the math and shader nodes as I have time. Unfortunately, I am currently in a very long meeting and can't post all the details at the moment.

Just as a teaser, the three techniques are:

1. Given the math behind partial transparency, calculate a negative ambient color that will properly darken the image with respect to R, G, and B. (Demo image here did it that way.)
2. Use a Refract node with IOR of 1 to "see" what's behind the shadow catcher. Blend that with a color, modulated by the shadow detector.
3. Use a Refract node with IOR of 1, but muliply that with a color, modulated by the shadow dectector.

I think #1 is fastest (no ray-tracing needed) but I suspect methods #2 or #3 will give more precise results by allowing more manipulation of the underlying image. I will experiment more and let you know.

Forgot to say, don't criticize the shape or angle of the shadow - you can work that out yourself. I was having trouble matching the degree of blurring with the rest of the scene.

Click to see full size and examine closely the variations in the ball's shadow. It is not a solid color, but rather is showing response to the variations of the real ground image.

WHOA!..

That is very impressive!

Well kyraia, how's your brain doing?,... Mine's about to explode. :) This is exactly the type of thread that keeps me coming back to this forum. Thanks, everyone, for all your ideas and inspiration. take care jdc

BB excellent , thanks for sharing and your time... going to test it out with the methods #2 and #3

Going to wait for a nod setup (although I am using Poser6, hope is the same), so I can see how this is achieved. Also, the sun is out, showing nice shadowing. Going out to take photographs. I am ready for some fun. 

Hi folks. Meetings, meetings, meetings. I'm about to go back in, so I didn't have time to develop and test methods #2 and #3. But, I'll present method #1 (using a negative color and transparency) for you. I will not explain the math, unless somebody asks for it. It's not too tricky, just basic algebra. But you don't really need to know it to use it.

I have designed the shader so that we can take all the guess work out of the process. Just follow the steps I outline and you should be all set.

Again, I'm not going to explain here how to generate the shadows in the right perspective. We can talk about that later. 

You can use a square or the ground plane for your catcher. I prefer a square as I can move it and scale it more easily. Here we go.

Load your background image into an image editor. Blur it. This is to remove details and measure the general color shift caused by light and shadow.

Then measure two spots that in real life would have similar colors if lighting was identical. Write these RGB values down. You'll need them later.

After calibration, if you change your lights, you may need to calibrate again.

The shader operates in two modes. These are controlled by the Calibrate node. A 1 here enables calibration mode. This is so you can render for shadow detector adjustment.

Do a render.

Note: 

I played with more lighting contrast, lowering my IBL and raising my SUN.

As a result, sometimes increasing Calibrate_WHITE will make the BLACK gray again. If that happens, increase Calibrate_BLACK a bit more.

Geesh....I guess I am out of this one. I do not have "Calibrate" nods. I also see "Shadow color" nods.

Using Poser 6. Do not have P7.

Dude those are math nodes. I used a plugin to rename them so you know what they are.

Just load the shader - all will be fine.

"I used a plugin to rename them.."

And which ones did you rename, and to what? Losing me here.

and where do we get this plugin to rename them? I could really use that... (or can they be renamed in notepad? runs off to see)

ooh you can use notepad... now to see which node is which..

http://poseworks.com/files/addons/WCRO_RenameNode.zip

aah thanks BB that'll save time lol

Quote - "I used a plugin to rename them.."

And which ones did you rename, and to what? Losing me here.

I renamed the ones that look like things you've never seen before.

Please don't be confused by the labels. Everybody thinks the label tells you what kind of node it is. It only does that by default, but I can change the label to something more meaningful.

You should not be worried about what the nodes are. Just load the shader, and fill in the values that I have labelled as such.

The shader needs to know the Shadow color - put that in the node called Shadow Color.

You don't need to know that it is actually a Simple_Color node. You need to know what it is for. That is how I've labelled them.

If you were going to try to build this yourself, you would need to know what types of nodes to add, but I really don't think you want to try to build this yourself, or try to change it. I haven't explained any of the math. Every time I try to explain the math, I get responses like "my head just exploded" or "this is too complicated for me to use - I'll pass". This is not complicated at all TO USE. 

It is WAY complicated to build, but I've already built it.

Just use it. Follow the instructions step by step. You will end up with perfect shadows.

Excellent work BB thank you ...and don't wonder for some response, not everyone is deep in this stuff.. but well you made the work here done so I see not really big deal to follow the screen..

Robo2010 my dear don't be lazy .. you can use it good in your next perfect manipulation!!!
I hope you made great shots the other day!