hdr floor fix method

[crackfox]

is there a way to mimic this technique in octane4blender?

https://www.youtube.com/watch?v=RsAUfQl ... efaultCube

[grimm]

Oh yeah, it's really easy. What ever you are using for the floor object (plane, etc), assign it a diffuse material and enable the "Shadow catcher" option on the node. Here I have a simple sphere and I'm using a plane for the ground. The HDRI is from pixar and called Luxo Jr.

[crackfox]

thanks grimm,

the shadow catcher is straightforward but i am wondering how to map for the groundplane to be perspecively correct.
most of time when i rotate the scene it becomes clear that the hdr perspective does not align with the floor.

the dude in the vid actually flattens out the bottom of the sphere and the mapping stays intact aligning really well with the object that is casting shadows. (around 3:20 mark)

i honestly hadnt tried it yet, in the middle of bunch of other things, but before i mess around for hours, thought i might ask for tips first.

[grimm]

Ah, I see, interesting technique. I can see why you would like to use this for animating with just a HDRI for the environment. I created a sphere just like in the video, but I wasn't able to get it to work. Mostly because Octane doesn't have an object mode that just shows the diffuse/albedo color without lighting considerations (at least from what I was able to find).

It's not really a problem with perspective, more it's an issue with how the HDRI is projected around the scene. The HDRI sphere is projected around the camera, so when the camera moves it takes the HDRI with it. This makes your objects in the scene look like they are shifting and scaling independently of the HDRI. You can see the simple plane I used in the image above is matched to the HDRI in it's perspective.

For still images I haven't had many issues finding a good position and scale to match the HDRI. Another option in this case would be to use something like FSpy to exactly match the camera parameters to the HDRI. You would need to capture a snapshot of the projected HDRI in the position you wanted first to run the match. This would allow you to build a more complex shadow catcher object that better matches the surfaces in the HDRI.

Unfortunately the only method that I know of for animation is that you have to build an environment with geometry and then just use the HDRI as the sky dome.

There might be another method that could work, which is to use compositing techniques with AVOs. I haven't worked with those much and you might have to wait for the plugin to get updated to the newer versions of Octane where I think that workflow is supported.

Anyways, you will have to be careful which HDRI you use with this technique. I tried it with the same Luxo Jr HDRI I used above and it didn't look very good. Looking at the two HDRIs he used in the video, he choose ones that were pretty much flat already and they had noisy surfaces that would help hide the projection warping. With Luxo Jr it foreshortened the ground too much and things looked very strange, in this case the lamp base and the steps for example.

[pegot]

This plugin allows interactions with HDRI ground via advanced shadow catcher but, unfortunately, only works with Cycles and Eevee

https://blendermarket.com/products/hdri-maker

[crackfox]

i appreciate both of your inputs on this, thanks.

yes, i really felt this is an interesting technique too. the problem with hdr is the spherical mapping at the bottom half, whereas this technique flattens the bottom into a plane while retaining the mapping somehow, like a dome.
i was just hoping there is a similar hack with octane.

obviously its also down to hdr but i always do my own. i find that while rendering out the hdr, camera should be placed roughly at the intended position (and not tilted in any way) where the camera is going to end up in the scene where hdr is used in.

i find hdr really helpful for cutting down the rendering time, especially when rendering volumetric heavy environments. i use 32k (photoshop limit) and manage to get good quality when cropped to the shot.
stills are fine ofcourse, its always this parallax-like effects that is the problem when rotating.

no biggie though, its easy enough to set a ground plane with materials and stuff.
would be nice however.

cheers.

[pegot]

I also shoot my own HDR Panos and am always looking for interesting ways of including them in my scenes.

I wonder if any of the OSL Camera projection scripts can help here? I'm not immediately familiar enough with them to know what to do or how to use them.