UH-60 Helicopter

Here is a really good use for motion blur. Unlike airplane propellers with a spinner, the helo rotor hub has a more complex structure that does not lend itself to being handles with a disk and transmap. The model is available free on animium.com

The website is a bit of a pain, sometimes I have to repeatedly pause and resume downloads when they freeze up, but it has quite a few high quality models free.

Wally

UH-60 Helicopter

Comments

Franz's picture

Re: UH-60 Helicopter

Hey Wally,

Very nice render. Indeed I love the motion blur on the head assembly.

Which version of appleseed did you use? If alpha-16, which pixel filter did you use? I can spot some minor aliasing here and there that would probably disappear with say a 3x3 Mitchell-Netravali filter.

It looks to me you used direct lighting only, is that correct? Out of curiosity, why?

Last remark: there's a slight black outline in the front of both fuel tanks. Any idea why?

Again, very nice work, as usual.

Franz

Re: UH-60 Helicopter

I used alpha-15. Just downloaded 16. Will try it soon. The fuel tanks were low poly and I did a quick postwork fix to smooth out the visible poly borders. Attached is a close up of the original render. I used no direct lighting. All lighting is by hdri environment map. You've commented on several of my renders that it looks like direct light to you. The only render I have posted that used any direct light was the steampunk spaceship, which used a direct light for the sun. Even in that scene, A large part of the illumination was an hdri for the sky light. I have always been puzzled by your impression of direct illumination. Of course, I can't look at my images objectively, I know how they were made, but it seems to me the indicators of indirect lighting are there. The complete absence of harsh shadows, and the presence of soft shadowing. On the helo, look at the way the illumination filters into the interior through the windows. On the fuselage is a box shaped object that shows occlusion shadows around the edges where it meets the fuselage. The center section around the doors is darkened due to soft shadowing from the weapons pylon. The pz11 aircraft render really shows AO like shadowing around the engine details, plus there is a large soft ground shadow. I wonder if you get the impression you do because I have been rendering objects with few concavities, which will show less of the AO like shadowing of GI. I have a collection of hdri probes and I spend some time trying different hdri enviroments and experimenting with the horizontal shift parameter to find a combination I like, so while I appreciate your encouraging comments on my renders, it is a bit disappointing that these renders give the impression of only direct light. Although I do not claim to have a great expertise in modeling/rendering (it's more of a hobby for me), I do know better than to light a helo surrounded by a daylight sky with 2 or 3 point or spot lights. I started out playing with Poser 6, and got into looking at other rendering options because I was not satisfied with the results of the direct illumination only option the native renderer offered. (GI is available in Poser 8, but it's not that good. The whole Firefly rendering technology just seems dated and in need of replacement). Maybe I'll render the helo with direct light to show the difference. The UH-60 render btw was illuminated with the beach probe from the light probe collection at http://www.pauldebevec.com/Probes/. I attached a preview. I converted this to lat/long format using hdrshop.

Franz's picture

Re: UH-60 Helicopter

Hey Wally,

The fuel tanks were low poly and I did a quick postwork fix to smooth out the visible poly borders. Attached is a close up of the original render.

Thanks. Are you using an Ashikhmin-Shirley BRDF on the fuel tanks? It turns out there was a bug that led to visible black outlines around objects silhouette. The bug is now fixed.

I used no direct lighting. All lighting is by hdri environment map. You've commented on several of my renders that it looks like direct light to you. The only render I have posted that used any direct light was the steampunk spaceship, which used a direct light for the sun. Even in that scene, A large part of the illumination was an hdri for the sky light. I have always been puzzled by your impression of direct illumination.

I think we are using slightly different terminologies here. An HDR environment map can be a source of "direct lighting" if the emitted light doesn't bounce on surfaces. Conversely, a simple point light can contribute to global illumination. It's all a matter of which light transport is used. Looking at this image, I'm under the (possibly wrong) impression that you used the Distribution Ray Tracing (DRT) engine (which doesn't compute global illumination). What makes me think so is the very dark (almost black) shadow below the right turbine's exhaust.

On the fuselage is a box shaped object that shows occlusion shadows around the edges where it meets the fuselage. The center section around the doors is darkened due to soft shadowing from the weapons pylon. The pz11 aircraft render really shows AO like shadowing around the engine details, plus there is a large soft ground shadow.

All these effects are either due to soft shadows / occlusion due to the very large light source. They don't necessarily indicate interreflections between surfaces (although there might be some that I don't see).

I have a collection of hdri probes and I spend some time trying different hdri enviroments and experimenting with the horizontal shift parameter to find a combination I like, so while I appreciate your encouraging comments on my renders, it is a bit disappointing that these renders give the impression of only direct light.

By all means, my comment wasn't meant as a critique, I think your renders look great, they are very clean and show a great deal of effort and exhibit a lot of very interesting effects. I was just wondering which lighting engine you were using, as some dark shadows seem to indicate a lack of surface-to-surface reflections. Again I might be wrong, and in any case the render look very realistic and well balanced!

In any case, please keep up with the excellent work. All your images are making it into the gallery.

Franz

Re: UH-60 Helicopter

Hey Franz,

I just checke my project file. Looks like I am using the unidirectional path tracer for the UH-60 renders. I have tried the distribution tracer. It is less prone to show firefly artifacts. I also noted some quite dark shadows as you mentioned. I was under he impression that by direct light you meant infinite/point/spot lights. Probably because the last renderer I spent a lot of time on was Art of Illusion (AoI) path tracer, and in that program all the code the term direct light was used to always refer to illumination from these types of physically unrealistic lights. That really is not the best way to use the terminology, as it is inconsistent with the meaning of direct vs indirect, and does not seem to be the conventional usage in the industry, but I got used to AoI speak. That clarification was helpful. I can see where the dark shadows give the impression of diffuse area lights without indirect bounces. I saw this type of image a lot when I used to use POVray. Although POVray is quite dated, I really enjoyed usig that ray tracer. I gave it up because although I could produce some nice renders using light domes, it's GI system was totally outmoded and more prone to artifacts than any renderer I have ever seen. I'm going to look into the dark shadows issue. Maybe it's partly the result of tone mapping. In the UH-60 renders, all the textures are PNG images (except the hdri light probe, which is openexr). I used linear_rgb (tried srgb on the helo textures, but the results didn't look as good). The images produced in the appleseed studio preview window look about right, but usually look a little low contrast to me. Generally I experiment with some tone mapping (mostly I have been using picturnaut). I'm going to subdivide those low poly regions so I don't have to post work it and experiment with more renders of the UH-60. Maybe I can bring up those dark shadows. I may render it with the distribution tracer to see the difference.

Wally

Franz's picture

Re: UH-60 Helicopter

I just checke my project file. Looks like I am using the unidirectional path tracer for the UH-60 renders.

OK. So I guess the black shadows are simply the result of tone mapping as you hinted later in your post.

I have tried the distribution tracer. It is less prone to show firefly artifacts.

Yes, because unlike the path tracer, it doesn't follow diffuse bounces (bounces on diffuse surfaces) and thus ignores entirely caustics on diffuse surfaces. In fact this is pretty much the only difference between the two lighting engines.

Maybe it's partly the result of tone mapping.

Probably. It would be interesting to see the non-tone-mapped renders. It would also be interesting to render closeups of these dark regions. Anyway that's just the curiosity speaking, the final image very looks good regardless of that.

I used linear_rgb (tried srgb on the helo textures, but the results didn't look as good).

Basically, if the textures "look good" in an image viewer on your monitor without any form of correction, then they probably are either gamma-corrected, or expressed in the sRGB color space. If they look too dark in an image viewer, they may well be expressed as linear RGB.

In principle, the right thing to do is to make sure the renderer uses linear RGB values for computation: that means marking as sRGB those textures that are in sRGB, so the renderer knows it has to linearize them before using them.

Typically, you also want to let the final frame use sRGB, otherwise it will be too dark. You can either do that in appleseed, or let appleseed render a linear RGB image and do the color space conversion / tone mapping in an external program.

The images produced in the appleseed studio preview window look about right, but usually look a little low contrast to me.

Do the renders displayed in appleseed.studio look any different from the one saved to disk? If so, that would be a bug.

I may render it with the distribution tracer to see the difference.

That would be interesting indeed.

Franz

Re: UH-60 Helicopter

I rendered the helo using the distribution ray tracer. There is an obvious difference in the interior light. The rest looks very similar, but an A/B comparison shows differences. I subtracted the two images and indeed there is additional light showing in the path tracer image. The distribution tracer image is more noisy because I rendered it with fewer samples. I also increased the brightness/contrast of the difference image to show the difference better. Images are attached. The renders are not tone mapped, just saved as png from the render preview window. It looks like the path traced images are showing indirect light, but the effect in this render is subtle.

The original textures look good viewed in an image viewer. I tried rendering with both linear_rgb and srgb settings on the textures (keeping linear_rgb for any bump/texture/alpha maps). The linear_rgb look good, but the srgb come out much darker.

Franz's picture

Re: UH-60 Helicopter

Very nice!

It's true that this kind of scene with ample illumination from the sky hardly shows the effects of global illumination, except of course in closed volumes such as the cockpit where incoming light from the sky bounces around and generates much stronger illumination.

Franz