osmosis | Study Hall - Interior tutorial

Tutorial: Study Hall Exterior
(lasted updated 16.01.2003)

[Download Scene]

This tutorial is a continuation of the previous Study Hall Interior, and uses the recently released VRay 1.0902q Advanced. It is not intended as a comprehensive tutorial on modeling, texturing or lighting of an architectural scene. The aim is to extend the Additive Lighting Strategy outlined in the previous tutorial to tackle an exterior shot of the same scene. In the process, we will also examine some other features of V-Ray that allow greater control and adjustment of individual objects or materials.

As this tutorial is meant as a continuation of the previous, I won't go into too much detail about things already covered. If you are starting this exercise from scratch, I suggest you at least review the Additive Lighting Strategy outlined in the Interior tutorial, and use that as a guide to setting up the Exterior shot. Ok, let's begin.

COLOR MAPPING
Preparation for the exterior is generally indentical to preparing for the interior, so refer back to the previous tutorial if you need to.

1. Switch to Camera04, which is located at the bottom of the stairs.

2. Leave settings as they were for the interior and render a preview of the exterior from Camera04. Make sure you have Advance Irradiance Map Parameters> Autosave and Switch to saved map checked.

3. This is a good start, but you'll notice that areas in direct sunlight are overexposed or "burnt".

In the real world, our eyes automatically adjust to cope with the contrast between areas of bright sunlight and areas in shadow. This is known as 'exposure control' or Color Mapping in V-Ray. The previous tutorial covered Linear Color Mapping, since it was the only option available at the time.

Prior to version V-Ray 1.0902q, Color Mapping was limited to Linear only. With the latest release, an Exponential option has been added which behaves in a similar way to Logarthimic Exposure Control in Lightscape and 3ds max5.

From the readme file:
Exponential Color Mapping mode: " You can select this instead of the default "Linear mapping". The Exponential mode is targeted at preventing burn-outs caused by very bright colors. The meaning of the Dark and Bright multipliers is slightly altered in this mode. The Dark multiplier can again be used to make the image darker/brighter. The Bright multiplier is not very meaningful at this point and you should leave it to 1.0."

4. Change Color Mapping Type to Exponential and rerender.
Since we are reusing the saved IR map, this should not require IR Map calculation.

5. The result with Exponential Color Mapping is much better. However, colours overall seem to be a little desaturated. I wonder whether this can be avoided...Also the background colour, in this case the colour of the sky, is affected by Color Mapping settings. It would be nice to have the option to ignore background.

MAKING ADJUSTMENTS
Although the Exponential Color Mapping helped reduce burn outs in most bright areas, the main concrete frame still is too overexposed, washing out any details in the concrete texture.

At this stage we have 4 options:
i. Color Mapping - Continue to reduce the Dark Multiplier.
However, this tends to reduce the overall brightness of the entire image. Details that are now visible in shadow may be lost. This is the only option that does not require a rerender.

ii. Bounce Multipliers - Tweak the Indirect Illumination (GI)
Adjust the Primary and Secondary Bounce Multipliers. This will require a recalculation of the IR Map. Again, the overall brightness and contrast of the image is affected.

iii. Object Settings - Assign specific GI Generate/Receive settings for individual objects or materials. This gives greater control over the amount of GI each object receives and generates. Notice that the underside of the concrete frames are alot darker than the original. Requires rerender.

iv. Sun & Environment Light - Reduce the multipliers of both the Sunlight and Environment light directly, thus affecting the amount of light in the scene. This allows greater control of the amount of illumination each light contributes to the scene. Requires rerender.

The final image above used a combination of these settings.
The values were set as follows:

Note: I also reduced the value of the Sunlight and changed the Receive/General settings for different objects. Many of these decisions are very subjective and depends on the look you are after. So the final image will depend greatly on what you think looks best.

POST PROCESSING
The final result from V-Ray is close but not there yet.
As mentioned earlier, using Exponential Color Mapping produces "washed out" colour due to the remapping of colour values. This is most noticeable in the dull colour of the sky.
And using Linear Color Mapping produces good colour saturation, but contains areas that are "burnt" or overexposed, especially on the main concrete frame structure.

So we'll try to get the best of both worlds by finishing off in Photoshop.

1. Render the final image using Linear Color Mapping.

2. Render another final image using Exponential Color Mapping.

3. Open both images in Photoshop.

4. Copy and paste the Linear-mapped image into a layer in the Exponential image.

5. Change the Blending Mode of the Linear layer to Saturation.
This forces the Exponential Layer to use the saturation values
of the Linear layer without altering the luminance values.

6. Drag the Exponential layer to the New layer icon. This creates a Duplicate Exponential layer. We will use this layer to Screen, or lighten, the dark areas in the image.

7. Change the duplicate layer to Screen blending mode. The entire image is lightened, including the areas in direct sunlight, causing them to be overexposed.

8. Use a Mask to control this Screen layer. Select all (ctrl + A) the image on the Screen layer. Copy (ctrl + C) the contents to memory.

9. Add a Vector Mask to the Screen layer. Go to the Channels tab.

10. Highlight the Mask channel and Paste (ctrl + V) the content from memory. The mask layer is a grayscale image that will cause all the pixels that correspond to black disappear, while white pixels will remain visible. What we want is actually the Inverse of the current mask, so Invert the mask channel.

11. Go back to the Layers Tab. Reduce the Opacity of the Screen layer to 50%.

The animated GIF on the left should illustrate the effect of each layer.

CONCLUSION
The interesting challenge in this tutorial was to create an image that had good dynamic range, without having areas of the image that are too dark or too bright.

In order to achieve this, we explored several different options and features in V-Ray that allowed us to fine tune the lighting solution.
Each option affected the image in different ways.

Color Mapping and GI bounce multipliers affected the overall illumination of the image.

Whereas Objects Settings and Light Multipliers allowed us to adjust the illumination of specific objects/materials or lights.

And although V-Ray produces some nice images, there are often limitations to what it can achieve. Many of these limitations can be overcome by using a good image editing program, like Photoshop. This allowed us to combine the desirable aspects of one image with the desirable aspects of another.

I hope you have found this tutorial useful and informative. We will continue to add other tutorials as time (and motivation) permits.

And as new builds of VRay are released, we will try to update the tutorials so that they remain relevant.

Please feel free to send feedback and suggestions to vu@osmosis.com.au.

Have fun and...I love VRay, do you?

Vu Nguyen|osmosis