An easy way to render only the ground shadows

monson67 · November 28, 2016, 07:19:44 AM

It's not hard to find discussions on the forums about using emissive materials to create renderings of only the ground shadows. This is the technique I currently use to do so. I also understand there is a lengthy method of using Photoshop to cut out the shadows using the clown pass and whatnot.

Yes, both of those techniques work, but neither one is very quick, especially if the model is complex when using the emissive technique.

What I would like to see is a "Render Ground Shadows Separately" checkbox in the output tab of the render window. Maybe it would only be clickable if Ground Shadows is turned on in the Environment tab. If the checkbox is checked, then KeyShot would render the image of the model without the ground shadows, but it would also render another image of just the ground shadows. Ideally, when rendering the ground shadows, it would render all of the shadows, including the areas that would be obscured by the model.

INNEO_MWo · November 28, 2016, 11:24:18 AM

Try to use a ground plane and put it on a separated render layer - like i n this example.

hope that helps

monson67 · November 29, 2016, 07:17:43 AM

Thank you for the suggestion. Though, for the work that I do, I need full ground shadows. In your example, the part of the ground plane that is obscured by the model doesn't show ground shadows in that area. That is why I currently use the tedious emissive-material method. I realize that I am probably in the minority in this regard. For when I don't need full ground shadows, I will use your technique.

richardfunnell · November 29, 2016, 08:12:52 AM

I don't know of a way to do exactly what you're hoping for, but when I set out to create a ground shadow pass, I use Scene Sets to make it much easier.

Create a Scene Set, call it something like "Shadows". You can use the in-scene material tab, filtering by visibility (or scene set), to quickly multi-select & link all the visible materials. After that, same process applies for rendering out the ground shadow pass, and the scene set is a nice way to isolate it. I use the same technique for creating new scene sets for each special pass needed.

Otherwise due to material reflections, global illumination, transparency, etc there's no pre-made way to calculate that based on your "normal" scene.

monson67 · November 29, 2016, 08:44:03 AM

Thanks

DMerz III · November 29, 2016, 01:17:34 PM

Hey Monson, here's a +1 from me. We would love this feature as well if it were possible.

For our process, we do exactly what Richard mentions, the scene set part does help us speed up time by a little. Link everything in the new scene set, and copy the "emissive shadow" material to all parts necessary. You can make it even quicker by saving this custom material to your library for future uses.

monson67 · November 29, 2016, 01:34:52 PM

Thanks for your replies, guys. Your suggestions should help me some.

Paul Garcia · November 30, 2016, 04:01:24 PM

Question....How do you create Sharp shadows....it seems like Keyshot is really good at doing soft shadows but I havent found a way for it to do a really harsh shadow, say a car on a sunny mid day shot....I have tried setting the contrast all the way up but doesnt get it to where I need it.

I end up doing my "harsh" shadows in Cinema 4D, it works great

Esben Oxholm · November 30, 2016, 11:44:09 PM

Quote from: Paul Garcia on November 30, 2016, 04:01:24 PM
Question....How do you create Sharp shadows....it seems like Keyshot is really good at doing soft shadows but I havent found a way for it to do a really harsh shadow, say a car on a sunny mid day shot....I have tried setting the contrast all the way up but doesnt get it to where I need it.

I end up doing my "harsh" shadows in Cinema 4D, it works great

Adding a really tiny pin with high brightness in the hdri-editor, a point light or an IES-light might help you get what you are after

DMerz III · December 01, 2016, 08:25:51 AM

Exactly what Esben said. If you were to use the sun and sky feature, you'll notice the sun is a very very tiny but extremely bright dot in the entire HDR "dome". Direct sunlight in nature creates the sharpest shadows, so if you emulate that with a pin, you'll get less diffused shadows overall.

It took me way too long to learn this, but once you think about it in that way, it made so much sense.

TLDR: Smaller point of light, but high brightness = sharper & stronger shadows.

Paul Garcia · December 01, 2016, 01:01:20 PM

Thanks Esben and dmerziii....I am going to play around with that idea

Paul Garcia · December 01, 2016, 09:50:59 PM

quick test for hard shadows....I am very happy. I am getting much better results now. The pin light is doing most of the work. The larger light is set to low power providing fill light. Thanks again Esben and dmerziii!

DMerz III · December 06, 2016, 08:00:29 AM

Hey! No problem! Glad to help, and like I said, once I discovered this, I had the same reaction!

Speedster · December 06, 2016, 08:50:29 AM

Are you rendering in "Interior Mode"? If so, try reducing any HDR Environment to zero, and use Maximum Time. I think it emulates an enclosed environment, and may harden the shadow edges even more.
Bill G

richardfunnell · December 06, 2016, 09:14:23 AM

Hi Bill,

I don't think this information is accurate, although the results may appear similar; interior mode doesn't emulate an enclosed environment without the geometry to contain the light.
What you're doing is simply losing the additional bounces that work very well for an enclosed scene, but in your example will give you very sharp lighting with the advantage of great looking physical lights.

It's not a bad suggestion, I simply want to make sure that it's an accurate description!