iray dev blog

Irradiance Buffer

Fri, 17 May 2013 10:01:09 +0200

In addition to the new LPE buffers we presented earlier, iray now also has another neat user buffer: The Irradiance Buffer.

Independent of the primary hit materials (i.e. textures, procedurals, layering, BRDFs, etc. that are used to describe a material), it contains the amount of (simulated) incoming light for all the points visible from the camera (which is not the most correct formulation for irradiance, but at least hopefully understandable :)).

Previously, hacks like replacing all materials with a purely white diffuse material were needed to get a very rough approximation of irradiance, but this breaks down easily as materials in the scene get more complex, a lot of transparency and refraction is involved and/or the amount of indirect lighting dominates the rendered view.

Note, that combined with the previous post on the Sun & Sky settings, this results in a pretty workflow to estimate illuminance in lux or foot candles (as an example: one gets around 100.000 lux when measuring this on a simple plane with date set to today and time to 13:37 (and around 20K for objects not being in direct sunlight)).

So here some examples, using a false color encoding (heat map):

Here the lux range is roughly 1k (only indirect / blueish) up to 100k (direct Sun & Sky / redish).

The range in this picture is roughly 0.02k up to 90k.

—

Carsten

Procedural Sun & Sky (and setting it up)

Thu, 02 May 2013 10:00:55 +0200

A change that slipped through mostly unmentioned is the procedural Sun & Sky. Previously, iray had to bake environment shaders upfront to support all kinds of custom shaders in the most compatible way to avoid manual replacements of shaders. Nowadays we try to support as much as possible natively in the core.

As a result, Sun & Sky can nowadays be changed interactively, but of course without loosing any efficiency in the various importance sampling techniques implemented inside of iray.

So now for the correct settings to work with luminance values that are mostly consistent with those of real world sun and sky light on a clear day (note that due to historical legacy reasons, the settings differ slightly between mental ray and the Iray Integration Framework (neuray)):

rgb_unit_conversion: 1.0, 1.0, 1.0
sun_disk_intensity: 1.0
physically_scaled_sun: on

multiplier: 0.31831 (mental ray mia_physicalsky) or 0.10132 (neuray sun_and_sky)

btw: In 3ds Max, use the Daylight System (Create -> Systems), set it to use mr Sun and mr Sky (Modify -> Daylight Parameters: Sunlight / Skylight) and change the multiplier in both to 0.31831. To make the settings perfect, one must also pick a matching tonemapper (as it is matched with the Sun & Sky settings internally): mr Photographic Exposure Control (Rendering -> Environment -> Exposure Control). Now change the Physical scale there to Unitless with a value of 1.0. Then simply tweak the exposure via Exposure Value (EV) afterwards, depending on time of day, indoor vs. outdoor scene, etc.

—

Carsten

Section Planes

Tue, 16 Apr 2013 15:10:00 +0200

As it has been a while since the last update (shame on us, but we have been busy implementing useful features :)), here a (technically) less spectacular, but rather popular and useful new feature:

Of course, multiple section planes can also be combined:

In addition, the section planes can either work as if they cut off the geometry completely (so let the light in as in the pictures above), or to just let the viewer take a peek inside, to take a look at the unaltered lighting.

(Fun fact: Actually section planes have been in iray pretty early already but then kicked out again, because nobody really used it back then)

—

Carsten

Instant Relighting & Nonphysical Effects

Wed, 05 Dec 2012 15:12:00 +0100

Today, we would like to give you a glimpse of one of the new features of our renderer.

Often, the image generated by the renderer contains all the effects you want, and looks roughly right. But you still may want to emphasize some areas, increase a highlight here, dim a caustic there, maybe tweak the color of a light source. You could modify your scene and re-render, but re-rendering frequently may be too costly. Image composition is often a cheaper alternative, and it lets you create effects which are not physically plausible. With iray, you can render different bits of information into different images. You can then use your favorite image manipulation software to tweak what you don’t like, without having to worry about breaking the things you do like.

Starting with iray 3.0 and the Iray Photoreal mode in Iray 2013, we support Light Path Expressions (LPEs), which allow you to do this separation. An LPE is a regular expression that matches some light transport paths that iray generates, but not others. Each result buffer can be associated with a LPE so that only paths which match the expression end up contributing to that buffer. Iray also allows you to render several buffers with different LPEs at the same time at almost no additional runtime cost. LPEs can distinguish between different surface properties such as diffuse or glossy, reflection or refraction, types of light sources, and names.

Some renderers use Arbitrary Output Variables (AOVs) to achieve a similar separation. While this allows access to some things that LPEs don’t give you, AOVs usually require modification of shader or material code. LPEs, on the other hand, can be used without modifying scenes or materials.

So, what do these expressions look like and how can they be used in practice? Take, for example, this image of a glass of whiskey illuminated by two light sources.

Let’s say we want to enhance the visibility of the caustic on the left and tweak the color of the rear light source. First of all, let’s get all contributions from the rear (environment) light source. We are interested in light emitted by the environment (“Le”) that bounces from any type of surface any number of times and then hits the eye (“E”). As in standard regular expressions, the dot character matches any interaction, and the star operator means “repeat any number of times”. So, we get “Le .* E” for our first buffer.

The other light is an area light, so we can filter for “La”. Or, we can filter for the names of groups of lights as well as individual ones. The caustics we are interested in have had any number of arbitrary interactions with the scene before hitting a specular surface and then ending up on some diffuse (or glossy) surface. The corresponding LPE is “La .* S (D|G) E”. Instead of “(D|G)”, we can also use “[DG]” or ”[^S]”.

We can filter all the other paths into the final buffer, for example, by specifying “La (.* (S | [^S] .) | .?) E”.

Now, we can modify and recombine these images in any way we want. 75% environment contribution, 110% area light caustics, 100% other area light contributions:

50% environment contribution tinted blue, 100% area light contribution tinted orange:

60% environment contribution, 30% area light caustics tinted orange, 100% other area light contributions tinted orange:

Depending on the operations we apply, the result is exactly the same as re-rendering with the corresponding operations on the scene. But it’s instantaneous and no re-rendering is necessary. In addition, we can achieve a look that we cannot get directly from simulated light transport.

Remember to disable tone mapping and gamma correction during rendering and apply it after compositing.

——

Daniel S.

Double Rainbow!

Mon, 15 Oct 2012 18:08:34 +0200

This bit of eye candy shows the magic of a thin beam of light being refracted through a (diamond!) prism.

——

daniel l

iray Next & Dispersive Caustics

Fri, 21 Sep 2012 13:46:08 +0200

Today, we have another great example for the improvements which are coming to the next major release of iray.

Here’s our test scene rendered with “iray classic” (what you would expect from iray 2.x):

And here’s the scene rendered with “iray next”:

This scene makes heavy use of subsurface scattering, caustics, and dispersion (those two gargoyles are made of diamond!). What about render times? The first image was rendered for about 2h00m, and the second for about 2h30m (using two Quadro 6000’s with 16 CPU threads).

Note that neither image is completely converged: we only rendered them to 95% completion to explicitly show the difference in noise and convergence behaviour.

cheers!

——

daniel l

iray Next: Sneak Peek

Mon, 10 Sep 2012 18:36:38 +0200

We’ve been working on improving iray to make it better at rendering difficult lighting conditions. The results are good so far, so we thought: how about a preview?

Let’s first look at the scene used in our post on frosted glass (http://blog.irayrender.com/post/19731699592/frosted-glass-part-ii). Before & after:

Those “shadows” are actually indirect transmissive glossy illumination.

Now let’s look at a more concrete example. Before & after:

Much of the illumination passes through translucent curtains, a pretty complex lighting effect which gets partly filtered out in the “before” shot. Also notice the reflective caustic casting an indirect shadow on the wall, behind the chair.

And finally, what happens when the sun shines on a wavy, polished floor?

The living room and bathroom models are courtesy of Autodesk, with content from Turbosquid. Thanks!

——

daniel

Shadow Acne and the Shadow Terminator

Thu, 09 Aug 2012 09:50:10 +0200

Looking at the title of this post you may wonder if the iray guys now completely lost their minds. But actually this is more or less scientific terminology for effects that each artist has (unfortunately) experienced at least once in his/her lifetime. iray (fortunately) solves one of them, but actually fails to solve the other, as do all other physically based rendering packages out there. So this post will try to explain why these problems happen and how to work around (or rather avoid) the second one.

Shadow Acne is usually what one can spot as dark dots on some surfaces in a rendered scene. While these will (usually) ‘vanish’ with an increased amount of samples, it actually is not part of the classical noise ‘problem’ that path tracing based renderers have to deal with. Instead it is the fault of numerical precision issues of the underlying rasterization or ray tracing implementation. A classic workaround is to expose a global epsilon/offset parameter that must be tweaked for each scene separately if one is unlucky (i.e. most users will have stumbled over this at some point). iray though will avoid this automatically without user interventions and scene specific settings, so the first part of this post ends with a simple solution. ;)

Unfortunately the second issue, the Shadow Terminator, is currently unsolved when using physically based materials within a path tracer. Also this problem will not go away with huge numbers of samples.

So what happens? Currently almost all renderers on the market will work on triangle or quad meshes at the end of the day. So even if the scene is completely setup using SubDivs, NURBS, or whatever your favourite brand is, the render engine will most likely repackage that into a triangle soup before being able to work with it. To avoid having visible edges/facets due to the triangulation of the mesh(es), one can (and should) use vertex/shading normals to smoothen these by interpolation during the rendering process itself.

One downside of this technique is that now the physical properties of the mesh are a bit jeopardized: While the ray tracing or rasterization will work on the actual, ‘physical’ triangulated mesh, most of the material calculations will be based upon the fuzzy interpolated normals. So, as you might have already guessed from this explanation, at some point there must be artifacts being introduced into the picture due to this, right?

And that’s not all, you can even make it worse: Simply assign a complicated bump or normal map to a material. This can (for some(!) scenarios) amplify this effect even more. Here a pretty extreme example:

The reason for this boils down to a simple explanation: One tries to manipulate physics by perturbing the original geometric normals of a mesh, which is something that the ray tracing/rasterization that has to rely on the original mesh can not automatically cope with under all circumstances.

Using classical shader based materials, there are some ways to work around this issue most of the time by careful implementation (for example some mental ray shaders can do this automatically) though, unfortunately this is not the case for all physically based materials, such as the BRDF models and the accompanying path tracing simulation used by iray (unless one is willing to pay the price of various other artifacts that could happen then).

Fortunately, the workaround is pretty simple: If you experience shadow terminator artifacts on a mesh, simply increase the triangulation, or in the case of bump/normal maps: use displacement maps instead (which looks much more realistic on mesh silhouettes anyhow).

——

Carsten

Texture Filtering

Mon, 06 Aug 2012 17:10:19 +0200

We received some questions in the past on (the lack of) texture filtering options in iray. With this post i want to shed some light on this and why we decided to not bother our users with such low level implementation details.

iray by definition doesn’t require to use common texture anti-aliasing techniques like MIP/SAT-mapping and trilinear filtering, or even view dependent techniques like anisotropic filtering techniques and EWA (=eliptical approximation to oversample the texture within a pixel/sample). The reason is very simple: As the image generation process in iray is completely based on path tracing, each pixel is filled with hundreds to thousands of samples, implicitly oversampling any texture that shows up in that pixel with the same amount. So instead of playing safe and pre-blurring the texture (what is basically happening when using one of the mentioned texture filtering techniques) iray delivers the best possible texture filtering automatically. In addition, iray does not even use bilinear filtering for the reconstruction of the texels, but a higher order scheme to increase the crispiness (especially noticable when using bump or normal maps) even more.

Simple Example:

mental ray using no filtering

mental ray using pyramid filtering

iray using “no” explicit filtering

——

Carsten

iray Material for 3ds Max

Tue, 24 Jul 2012 18:53:00 +0200

This blog has seen several posts on the capabilities of the iray material model. So far we’ve shown:

Metallic flakes
Sub-surface scattering
Thin film coating

And there’s a lot more that we haven’t shown.

Today, life gets a little more wonderful for all 3ds Max users: we’re finally introducing the iray Material plugin for 3ds Max (2013).

The plugin is designed to be powerful, simple to use, and iray-friendly. Take a look at what a programmer can do in a couple minutes:

… and imagine what you could do!

You can download the plugin from link posted our public forums:

http://forum.mentalimages.com/showthread.php?10493-iray-Material-for-3ds-Max&p=40449#post40449

happy rendering!

daniel

P.S.: The image above shows, from left to right: metallic flakes, thin film coating, and backscattering.

Thin Film & Soap Bubbles

Mon, 16 Jul 2012 13:59:39 +0200

Here’s an image I rendered for EUE 2012, showing off iray’s thin film coating feature. The bubbles are made of simple spheres (deformed manually). The material is basically water, but with the “thin film coating” feature enabled.

The bubbles reflect an HDR environment map (and each other!). I placed a dark green object behind the bubbles, somewhat mimicking foliage, to increase the contrast: the color hues would otherwise be much more subtle.

——

daniel l

Matte Object Example

Fri, 22 Jun 2012 15:01:00 +0200

Here’s an animation created using iray matte objects:

The scene was created by combining this backplate photo:

with three water-filled glasses and two miniature cars. A chrome-ball image was used for environment lighting.

This example demonstrates the panoply of interactions, between synthetic and matte objects, which iray is able to simulate:

reflections
refraction
direct and indirect illumination
depth of field
motion blur

All this is possible because iray assigns regular/full-fledged materials on matte objects. The harder part is to make the materials match, but that’s actually fun!

PS: The car model is a courtesy of Autodesk, thanks!

——

daniel l

iray and product rendering

Tue, 22 May 2012 10:02:27 +0200

In this blog entry I am going to write about adjusting materials in iray. Since I work a lot with audio hardware in my spare time I recently decided to model an audio mixer that would be too large for my home studio. But being a 3d model, it doesn’t consume any physical space and I can take virtual pictures and send them around as if it were my own.

Modeling

I wanted the audio mixer to look as natural and real as possible, so using iray for rendering was the way to achieve that goal. When modeling a piece of hardware it pays off to think first at which detail level you intend to render the model.
I wanted to render great looking close-ups, so I spent more time with modeling detailed parts, such as beveled edges for the screws and adding more bevels for the knobs.

Figure 1: The wireframe and the rendered screw. the fine bevel shows nice thin reflections on the bevelled edges.

This adds a lots of polygons, but in areas of interest you really should have a high enough tessellation. This is especially valid if you want to render macro shots. Hard edges tend to look unnatural. If you never show areas in close-up than you can save work and polygons.

Textures:

Initially I wanted to put all the labels on the mixer using a 6k x 6K image for all the fine print on it. However even this super large texture was too small to look good in close up renderings. Apart from that a texture of that size consumes enormous amounts of memory. Instead I drew the entire labels in a vector program and converted the labels to a polygon mesh. This geometry was put with a slight offset on top of the mixer. This helped to make all print also looks great in a close-up renderings.

Materials:

When it came to assigning the materials to the geometry I relied on my eyes to achieve the desired look. I looked at real world photographs to quickly adjust the iray materials. Depending on the material you want to model, it is simple to take a look around the table that you are currently sitting at. Try to find an item on your table that has similar material properties to the material that you are trying to model. Then view it from different angles and see how it is reflecting the light and how blurry the reflections are. I typically take the item and hold it against a light source to achieve this.

With the previously made observations in mind I then adjusted diffuse color, reflectivity and the glossiness of the reflectivity to get the look right.

For the plastic knobs, I decided to add a noise-based micro-structure. When you look at a fader knob from very close, perfectly smooth surfaces look too artificial. Therefore I used procedural noise to create the rough bumpy plastic surface which really adds a lot to the look of the final renderings.

Figure 2: The noise based bumps on the fader knobs enhance the realism, especially in close-up renderings

Figure 3: Different iray renderings of the audio mixer. Note how irays depth of field adds to the photographic feel of the renderings.

PS: All the images above are rendered.

——

Rüdiger

End User Event 2012

Fri, 18 May 2012 14:41:03 +0200

I’ll be at EUE in Utrecht in two weeks, to provide some tips, insight on the iray roadmap, and show off iray’s latest production features. Don’t miss it…

http://www.enduserevent.com/

——

daniel l

iray Matte Objects in 3ds Max 2013

Mon, 23 Apr 2012 18:04:27 +0200

Our last post introduced the matte objects feature… Today, we’re showing you how to enable and use it in 3ds Max 2013.

Disclaimer

The feature we’re about to demonstrate isn’t official in any way. It’s neither exposed nor documented in 3ds Max 2013. It received very little testing and end-user validation: it’s still in beta stage. While we invite anyone interested to try it out, it should be understood that we can make no promises with regards to stability or the continued availability of this feature in future releases of 3ds Max, in this form or another (official or not).

That said, we sure hope you’ll appreciate the feature as-is, and perhaps help us make it better!

1. Exposing the Feature

The user interface is exposed through a mental ray environment shader. The first step is to grab the definition file, “iray_matte_environment.mi”, from our forum:

http://forum.mentalimages.com/showthread.php?9454-iray-Matte-Objects

Then copy the file to the following folder:

<3dsmax2013_installation>\NVIDIA\shaders_autoload\mentalray\include

For example:

C:\Program Files\Autodesk\3ds Max 2013\NVIDIA\shaders_autoload\mentalray\include

And re-start 3ds Max.

2. Activating the Feature

Open the Environment panel and change the environment shader to: “iray Matte Environment”. That’s the shader we added in step 1. If you can’t find it, you may need to enable the “Show Incompatible” option in the material/map browser (or ensure that iray or mental ray is selected as the material editor renderer).

Drag the new shader to the material editor to edit its parameters.

Make sure that “Enable Matte Objects” is checked.

3. Specifying an Environment/Backplate

An environment and/or backplate map needs to be specified to get the full benefit of matte objects. They define the appearance of matte objects when they interact (through reflections/refractions) with synthetic objects. The backplate is projected onto the matte object from the camera; if the projection is impossible, the environment map is used instead. If a hemispherical environment of sufficient resolution and quality is available, it can be used alone. But these are difficult to generate, so in most cases a 2D photograph will be used as the backplate, which is basically a high-quality complement to the environment map.

It’s possible to use the matte objects feature with neither a backplate nor an environment map. In such a case, matte lights should be used to approximate the scene lighting. But many interesting effects will be missing from the rendered image, such as reflections of matte objects onto synthetic objects, because the backplate/environment defines how matte objects appear in those reflections.

4. Flagging Matte Objects

Matte objects and lights are flagged by adding them to the “Matte Objects” and “Matte Lights” lists. If you’re wondering about a “Matte material”: it’s not needed. Simply use regular material on all matte objects (Arch&Design, Autodesk Materials, etc.). Refractive and emissive materials are valid, too.

5. The Ground Plane

iray can render an infinitely large, procedural ground plane which behaves as a matte object. It’s a quick and easy way to add ground shadows and reflections. The “Ground Plane” rollup exposes these parameters:

Enable: enables the procedural ground plane
Up Direction: a vector which controls the orientation of the plane
Height: position of the plane relative to the scene origin, along the up direction
Reflectivity: how reflective is the ground plane
Gloss exponent: how glossy is the ground plane, with 0 being specular and higher values being more diffuse

6. Other Parameters

The remaining, more advanced parameters exposed by the shader are:

Render for Compositing: the rendered background is forced to be black. This allows for compositing the actual background in post, using the alpha channel which always accounts for matte interactions. The backplate is still used when a synthetic object interacts with a matte object, so it’s a good idea to specify one nevertheless (even if approximate).
Matte Shadow Color: artificial shadows, created on matte objects and the ground plane, will be tinted with this color. It’s useful when a non-white ambient lighting is present in the scene.
Environment Shadow Mixer: changes the relative weight of the environment and matte light sources when computing artificial shadows. At 0.0, there are no environment shadows; at 2.0, there are no light source shadows; at 1.0, they have equal weight.
Light Color Affects Shadows: when enabled, artificial shadows take into account the color of the light from which the shadow is generated (a yellow light will cast blueish shadows).

Note that all parameters have a useful tooltip.

Discussion Forum

For any comment or discussion related to this feature, please join us on the NVIDIA ARC forum:

http://forum.mentalimages.com/forumdisplay.php?156-mental-ray-iray

Enjoy!

——

daniel l

Matte Objects in iray

Wed, 18 Apr 2012 16:15:11 +0200

Today we’re introducing the latest iray feature to be released: matte objects.

Overview

Let’s start with quickly going over what matte objects are all about. Say you have a photograph (which we’ll call the backplate):

… and you want to add some synthetic (computer-generated) objects to it. In our case, we want to add some lumber that’s been left behind on the pier. The obvious starting point is to model the lumber and drop it in the scene, where it seems to fit:

… but that’s not right, the lumber is floating in space! And is the right part in front or behind that wooden post? This is where a renderer’s matte objects feature comes in. The goal of such a feature is to faithfully combine synthetic objects with the backplate, to make it appear as if the whole scene is part of a single photograph. So let’s walk through the setup required to achieve this with iray…

The first step is to create matte objects, stand-ins which are modeled and positioned to match, as closely as possible, a select set of elements from the backplate. Whichever objects you think might interact significantly with synthetic objects need to be modeled in this way. In our simple test case, we chose to model the pier and wooden posts around our lumber:

Note how the matte objects were assigned materials which matched the backplate as closely as possible. We’ll get back to that later. The next step is enabling the matte feature in iray and flagging each of the matte objects. Then hit render and:

Like magic, the result is a perfect* composite of the synthetic object in the matte scene.

*disclaimer: this is developer art!

Some Details

The classical approach to supporting matte objects involves “matte/shadow materials”. Such materials use a myriad of tricks to add specific, but limited, effects to the backplate: shadows, ambient occlusion, some reflections and illumination. iray’s implementation parts with the classical methodology, instead allowing any regular material on matte objects. The only requirement is that they match the backplate as closely as possible. iray will then render the scene normally and appropriately integrate interactions between matte and synthetic objects. The light path and material evaluation logic are unchanged, enabling the full set of lighting effects to be present, either direct or indirect: reflection, refraction, diffuse, glossy, specular, area lights, IBL, etc.!

Let’s elaborate on our sample scene:

We added an additional matte object for the gravel part of the ground, and two new synthetic objects: a big and a small SUV (with ridiculously powerful headlights). The goal is to demonstrate the interaction of synthetic light sources with matte objects. Any light generated by a synthetic objects, whether directly by a light source or indirectly by bouncing on a synthetic object, will light the matte object normally and be added to the backplate.

Let’s further tweak our scene to demonstrate how indirect lighting interacts with matte objects. We use a few reflective planes to illustrate interesting effects. The observations we’ll make are perfectly valid for all types of indirect illuminations, as there are no significant differences between glossy, specular, and diffuse reflections for iray.

We see that reflections of matte objects are consistent with what is directly visible to the cameras. The back side of matte objects are approximated by using the front-side projection, typically yielding very acceptable results, especially if one is mostly interested in diffuse indirect illumination.

Summary

Let’s wrap this up by outlining some the advantages of iray’s matte object implementation:

Very simple to set up
Arbitrary materials on matte objects (even transparent/refractive!)
Light sources can be matte objects, too
Full light path simulation, no cut corners

We’ll have more for you very soon, stay tuned!

PS: The SUV model is a courtesy of Autodesk, thanks!

——

daniel l

iray on Kepler

Tue, 27 Mar 2012 11:49:00 +0200

Kepler, the new generation of NVIDIA GPUs, is now shipping within the new GeForce GTX 680 card… But an important note to avoid surprises: iray doesn’t run on Kepler GPUs (yet!).

The binary code for this new GPU generation is different from the previous one, so you will need a re-compiled version of iray for Kepler-based chips. Rest assured that we’re working with your favorite software vendor to get you an updated iray as quickly as possible. But please be patient, it will take a little time to get these through the testing and release process. So stay tuned!

——

the iray team

Frosted Glass, Part II

Thu, 22 Mar 2012 15:21:05 +0100

In today’s installment, we take a closer look at the shadows cast by frosted glass objects (or any glossy transparent material).

The image above re-uses the setup from our previous post: a Sun & Sky, a cylindrical light source to produce highlights, and a few high-tech teapots. We see five glass panes which get progressively more frosted: from specular (glossiness 1.0) to very frosted (glossiness 0.0). The problem that immediately strikes one’s eye is the discontinuity in shadows: one might expect the 0.99 case to produce a shadow nearly identical to the 1.0 case, yet it ends up nearly as dark as the 0.0 case. Why? Let’s shed some light on this…

The shadows cast by glossy refractive objects could be considered a form of caustics. Whereas a caustic typically involves converging light rays, frosted gloss involves divergence; but from the renderer’s perspective, these are also difficult to simulate. While iray simulates all types of light paths in a physically accurate way, it’s better at some than others. To avoid introducing too much noise where difficult effects are present, we implemented some special-purpose heuristics which (automatically) filter out those difficult effects to produce a converged image faster. That’s why difficult caustics will usually be missing or much dimmer than one might expect. The same is true for light which traverses the frosted glass in the image above.

To demonstrate these heuristics, let’s see what happens if we make it easier on iray through an increase of the sun’s radius of 2.5x.

The shadows are now much closer to what we might have expected. A larger sun (and large lights in general) tends to simplify the lighting simulation. The heuristics react (automatically) by filtering less. This is one of the secrets behind our push-button paradigm: no need for quality knobs because iray knows what it can and can’t do. As we improve our algorithms, iray will automatically adapt and resolve difficult effects with more quality and precision.

——

daniel l

"Zippering" and Banding quick update

Tue, 13 Mar 2012 17:20:14 +0100

As some people seem to have serious zippering or banding issues currently that are hard to come by without changing the look of the specific scene, as a quick workaround, try to use the following clever “trick” by Jeff Patton:

Adding a noise map to one of the slots of an affected material can succesfully randomize the patterns/banding, i.e. you replace it by a bit of noise.

Thanks a lot Jeff for pointing this out!

Frosted Glass, Part I

Tue, 13 Mar 2012 10:00:06 +0100

Frosted glass is a hot topic as many users seem to struggle with rendering it realistically. Today’s blog post looks at different ways to create frosted glass.

The following picture shows four frosted glass panes rendered using three different methods. The scene is lit with a Sun & Sky system and a cylindrical light (the white horizontal bar) is used to emphasize reflective highlights.

Before we go any further, let’s define what we mean by “frosted glass”. Frostiness is, for our purposes, a surface feature created through a process like sandblasting, essentially an infinitely fine bump map. When setting up this material, it’s crucial that reflective and refractive properties match perfectly, otherwise we’ll get both visual and mathematical inconsistencies. We’ll see why…

The first pane on the left uses translucency and diffuse reflection with Fresnel blending. This simulates a very finely sandblasted glass. Translucency is a rather simple effect for iray to render, so this is a good choice if the look is what you want. But if you need something rougher, you’ll need to use glossy reflection and refraction.

The four other panes use glossiness to achieve a frosted glass look which is less pronounced. We have:

0.5*: refractive glossiness = 0.5, reflective glossiness = 1.0
0.5: refractive glossiness = 0.5, reflective glossiness = 0.5
0.0*: refractive glossiness = 0.0, reflective glossiness = 1.0
0.0: refractive glossiness = 0.0, reflective glossiness = 0.0

When creating a frosted glass material, it’s natural to think in terms of refractive glossiness. But, as stated earlier, frostiness is actually a surface property. It’s therefore important to assign identical glossiness values to both reflection and refraction, for two reasons…

The first reason is rather obvious: specular reflections on frosted glass are unrealistic. Such materials don’t exist in the real world, unless of course the glass is only frosted on one side, or if the frostiness is a subsurface effect.

The second reason is more mysterious, but it’s responsible for making the 0.0* pane very dark. Setting a material to different reflective and refractive glossiness values is like specifying different surfaces for reflection and refraction. Which of these surfaces should the renderer then use when calculating refraction or total internal reflection angles? There is no correct solution, the material simply isn’t physically plausible, so iray makes an educated guess. But to avoid mathematical Neverland and artifacts, some paths have to be discarded. The result is energy loss which gets worse with lower glossiness values.

You might also notice that 0.5 seems less blurry than 0.5*. This is just a side effect of the difference in behavior. It’s a nice example for how things get out of hands when we have to deal with non-physical materials.

Stay tune for another post on frosted glass…

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daniel l