Shadow Catchers for Newbies

[voon]

I don't quite seem to understand it. I have a spaceship model ((C) CIG, not mine) and put it into a grassy HDRI. I wanted to try out shadow catching and have put a plane under the ship, fairly large, extends to about 4-5 times the length of the ship. I assigned a standard diffuse material to it and activated the matte option .. changed nothing else. Now theres not much direct light in the HDRI ...

But still: I get very, very little shadow ... especially around the front wheel it loos weird? I tried out much sunnier HDRIs ... desert roads etc ... still: no shadows ... or very very faint ones. What am I doin wrong, please?

I was using the blender plugin btw which is still Octane 1.55.

[gordonrobb]

First thing I would do is check your hdri is good enough to cast a decent shadow (they're not all good enough). Without matte option mo, is there a shadow cast on your ground object? If not, either your hdri is not good enough, or the ground object isn't directly below your ship. Also, try using Daylight system to see if that works.

[FrankPooleFloating]

Yeah, HDR is first place to check, like Gord says. To expand just a little: With a really good HDR (proper daylight for hard shadows), you should be able to crank down your HDR about 10-18 stops in Image > Adjustments > Exposure (in PS) before the sun totally disappears, and it should be just a faint dot when it does. The very best HDRI don't go completely black until high teens. If ever there was a perfect example of "Not all ____ are created equal", it would be HDRIs...

[TBFX]

Also a common issue with HDRI's that are taken in dappled light/tree shadows like the one in your pic is that they have been taken in a shadowed area of the scene which means the light from the sun isn't recorded at all even though it is lighting things in parts of the HDRI, this makes them useful for lighting in the shadowed areas of the scene only.

I'm not entirely sure this is your only problem though as you mention trying other HDRI's but I would defer to gordonrobb and FrankPooleFloating's comments above about the inequality of HDRI's out there as the first thing to check.

T.

[voon]

What is a "good" HDRI? I'm a bit confused, because up to now, I consider a HDR image to be merely aa foto with a high dynamic range, i.e. no areas drowning in shadow and no areas exploding in white. The image on my picture is a simple JPG out of a free HDRi package. There's also a file labelled .hdr, but that seems to be less resolution (JPG was called 8k, the HDR 3k). I assume, that the HDR environment is just a texture emission ... i.e. each pixel just emitting light in its color ... basically just a colorful huge spheric lamp around the scene. Is this wrong? In that context, I'm not clear on what a "good" HDRi is ... do you mean images, that contain a strong bright light (liek the sun itself) etc?

[mib2berlin]

do you mean images, that contain a strong bright light (liek the sun itself) etc?

Yes, with Octane 2.0 you can mix HDR and daylight system, may give this a try.

Cheers, mib

[gordonrobb]

If you're using a jpg, you will definitely not get a shadow. Jpgs in these sets are designed for backdrop only, not lighting. You need to light it with the HDR image. If you're not up on compisiting the result, then I would find a better HDR. The quality of an HDR is defined by the number of exposure brackets used to create it.

But, use the daylight system instead of HDR first, to test wether your shadow catcher is working. Then worry about your HDR.

[resmas]

I don't quite seem to understand it. I have a spaceship model ((C) CIG, not mine) and put it into a grassy HDRI. I wanted to try out shadow catching and have put a plane under the ship, fairly large, extends to about 4-5 times the length of the ship. I assigned a standard diffuse material to it and activated the matte option .. changed nothing else. Now theres not much direct light in the HDRI ...

But still: I get very, very little shadow ... especially around the front wheel it loos weird? I tried out much sunnier HDRIs ... desert roads etc ... still: no shadows ... or very very faint ones. What am I doin wrong, please?

I was using the blender plugin btw which is still Octane 1.55.

Just to be sure.

Your plane (diffuse with matte option enable) have correct the normal direction?
If the plane has its normal to the wrong direction it will not receive any shadow.

cheers
resmas

[riggles]

What is a "good" HDRI? I'm a bit confused, because up to now, I consider a HDR image to be merely aa foto with a high dynamic range, i.e. no areas drowning in shadow and no areas exploding in white. The image on my picture is a simple JPG out of a free HDRi package. There's also a file labelled .hdr, but that seems to be less resolution (JPG was called 8k, the HDR 3k). I assume, that the HDR environment is just a texture emission ... i.e. each pixel just emitting light in its color ... basically just a colorful huge spheric lamp around the scene. Is this wrong? In that context, I'm not clear on what a "good" HDRi is ... do you mean images, that contain a strong bright light (liek the sun itself) etc?

At a basically level, yes, and HDR is an image with high dynamic range. Since a JPG does not have high dynamic range, you should not light with it, as mentioned. An HDR can contain very high and very low color values (or lumince values if used for lighting). This allows for very high contrast in certain areas, like the sun, which result in darker and sharper shadows. Using a JPG, which is an LDR low dynamic range image, those values are flattened and result in low-contrast or even "muddy" renders.

I think of HDRs like looking at the sun, and LDRs like looking at a picture of the sun. When lighting, you need the real thing.

What constitutes a "good" HDR is subjective. They're not all created the same. One person can create an HDR from three photograph exposures and call it a day. Another will use 8, 12, or even more. The more exposures, the more range the HDR has. This can not only give you sharper shadows, but it also gives you greater flexibility with different lighting intensities. Bringing the power down on a high quality HDR will still give smooth results, whereas a low quality one may not work well.

[voon]

Hm ... so can I somehow use the JPG as background, but the HDR supplied as light source? I'm not sure how to do this.