As we all know, post-processing is a major component in producing high quality renderings. V-Ray has an integrated compositing system which is invaluable to any 3d artist using many of today’s popular image editing software packages. This tutorial will introduce you to the channels of V-Ray, and how easy it is to use them to control key aspects in the rendered image including: lighting, reflections, and refractions.
When V-Ray renders, it calculates several of the different properties of materials and lights individually and composites them together to form the final image. By default, V-Ray only exports the final composited image. What we will be doing in this tutorial is telling V-Ray to export these properties as individual image files which we will be able to layer and tweak to form the final image ourselves.
There are a couple options we need to change in order to have a proper set of images to work with. Go to your V-Ray for SketchUp Options Editor and expand the Color Mapping options.
Ensure that you have “Don’t affect colors (adaptation only)” checked; This will tell V-Ray to not add the gamma correction to any of the exported images. Next, make sure that the option “Clamp output” is not checked.
Now expand the “VFB channels” section.
Here we will be selecting which channels we would like to render out separately. Leave the default channel “RGB color” selected, and make sure that the following channels are also selected:
Recommended: It is always a good idea to select an output file before rendering. Go ahead and expand the “Output” section and enable “Save output”. For file format, regardless of what the final image format will be, it is always best to use a 32-bit format. This ensures that the least amount of rendered information will be lost during post-processing. .JPG and .PNG are both 8-bit formats, while .TIF is a 16-bit format and .EXR is typically a good choice as it is a 32-bit format.
Finish tweaking settings for your final render, then hit the “Render” button.
While the image is rendering, you can enable the “Display colors in sRGB space” button at the bottom of the “V-Ray frame buffer” window. This will show a preview of the image with the correct gamma settings that we disabled by checking the “Don’t affect colors (adaptation only)” option. We can also cycle through the channels we selected earlier by using the drop down in the upper left. You can see that the RGB color is on by default. Note that the other channels will display as black until the final pass.
Part III: Explanation of different channels
While we are waiting for the render to finish, let’s review what the channels we are exporting mean. These channels make up the core basic elements that make up any V-Ray rendering. They can be further subdivided, but that is for another tutorial.
GI (Global Illumination)
This layer shows the secondary light bounces.
The first light bounce is covered in this layer
This layer is the image bounces off of any reflective materials in the scene. Any reflections that are not blurry show up in this channel.
This layer shows the light that is refracted through any materials that are set up to do so.
This covers light sources that are shining directly into the camera. This includes emissive materials as well as lights.
This is second reflective channel which handles blurry reflections. When you set the glossiness value of a material to be less than one, blurry reflections are produced which will appear in this channel, while the less blurry portion of the reflection will show up in the reflection channel.
Part IV: Export the image files
If you have not set an output file location before rendering, you will need to save the files from within the frame buffer once it has finished rendering. Make sure to use the “Save all image channels” button shown below. This will save your RGB Color channel using the filename of your choice and every additional channel will have a suffix with the channel name appended to it.
As mentioned above, it is best to use a 32 bit format such as .exr which will have more information in it allowing for for a higher-quality end result, even if you plan on saving the final image out as a lower quality version such as a .PNG or .JPG.
It is usually best to ensure that the files have saved out before closing the V-Ray frame buffer.
Part V: Opening the images in Adobe Photoshop
For this tutorial, we will be using photoshop, but other Image editors, such as GIMP, can also be used.
We could open each file individually and then combine them within photoshop manually, but there is an easier method. In Photoshop, go to File>Scripts>Load Files into Stack… Now click Browse and select all of the files you exported. You do not need to include the RGB color file, but it can be nice to bring it in to compare with the final composited result.
Go ahead and select “OK” and your files will be loaded into the project.
From here, hide your RGB Color layer (the one without a suffix). Select the remaining layers, and using the Blending Modes drop down, change all of these layers from “Normal” to “Linear Dodge (Add)”.
You now have the completed image. If you are using .EXR files, your image will look exactly as it should. With some formats like .TIF and .JPG, you will need to add an exposure layer for the whole image with a Gamma Correction of 2.2. Again, this is only a ne
cessary step if you did not use the .EXR format.
At this point, we can start each layer individually to change the final look of the image. Lets make a few basic modifications to improve the look of the image.
For starters, the reflection is causing some areas to look a little overbright. Rather than re-rendering the image, we can just turn down the amount of reflection. To do this, select the Reflection layer in Photoshop, and change the Opacity to something around 40%. You will notice that the reflections in the entire image have diminished.
Next, let’s dark the black areas on the image on the tv. We used a rectangular light to produce the image, so it resides on the Self Illumination layer. One way would be to adjust the brightness and contrast of the layer, but the better method is to create a new exposure layer that only affects the Self Illumination layer. This will allow us to edit it as much as possible without diminishing the quality of the original with each edit. To do this, first create an Exposure layer as explained in the last part of section 6, and drag it to be directly above the Self Illumination layer. Next, select the new exposure layer and in the properties, select the option which clips it to the layer below, meaning it will only affect the layer directly below it. This option is highlighted in yellow below.
Now, using just the Exposure and Offset sliders, we can modify just the Self Illumination layer. I used an Exposure value of -0.40 and a Gamma Correction value of 0.43.
Some of the areas in our rendering are a little overbright. We can use another exposure layer like we used on the Self Illumination layer. Clip it to the Lighting layer to correct this. Set the Exposure value to -0.40 and the Gamma Correction value to 0.70.
Another useful tip, especially when working with exterior renderings with underdeveloped interiors, is reducing the opacity refraction layer. If we turn down the opacity of the refractive glass, we are left with the diffuse and reflections. The result is a tinted glass.
Part VII: Saving the file image
Once you get the file to look how you want, you can save out the image to the format of your choice. If you are using .EXR or any other 32-bit format, you will need to convert the file to 8 or 16 bit before you can save it out as certain compressed formats such as jpg or png. The recommended path is to save the file in 32 bit format as a .PSD first in case you wish to edit the image later. Next, to save as a smaller-sized .JPG or .PNG, go to Image>Mode>16 Bits per Channel or 8 Bits per Channel. Hit the “Merge” button, and select “Exposure and Gamma” for the Method.
This will remove all layer information from the file, so make sure you do not save over your .PSD file at this point. You can now save in other formats.
This concludes the introduction to channels tutorial. Using these basic channels, you will be able to more easily separate renderings for post-processing to produce better results faster.
Final result with some vignetting added: