Step 7: Render layers
For my render layers I created a few simple passes that I would later composite in Photoshop. I made a beauty pass, a dust pass, a metal/rust pass, an occlusion pass and material ID passes.
The beauty pass
The beauty pass is the overall color and base materials of the vehicle. This pass will be the base upon which you will layer everything on top of.
This shows the beauty pass
The dust pass
The dust pass is made by adding a dust material to every part of the vehicle. This pass will be masked out completely and you can paint into the mask to reveal the dust where you want it to show in the final composite.
The metal/rust pass
The metal/rust pass is the same, but only added to the metal parts of the vehicle. This pass will also be masked out and painted into the mask to reveal where you want the metal and rust to be shown.
This shows the metal/rust pass
The occlusion pass
The occlusion pass is put on top of all the layers and changed to multiply to help define where the ambient shadows are in the render.
This shows the occlusion pass
The material ID passes
The material ID passes are used for quick selection of different materials for use in Photoshop. I like to use only red, green and blue colors in my material ID passes as this gives me automatic channel selections in the RGB in Photoshop and makes for quicker selections.
This shows the material ID pass
Step 8: The background image and importing the model
After all of your render passes are completed it is time to combine them in Photoshop. I started off by searching for a background image to place my vehicle in. My background image was from Wikipedia
(and is under the Creative Commons Attribution-Share Alike 3.0 Unported license).
Next, I adjusted the image a little, moving mountains around and adding a few more mountains to the background to make it a little more interesting to look at and fit in the scene better with the truck.
Then I brought in the base render of the truck over the background image and placed it where I wanted it to fit in the scene. I created the dirt road for the vehicle by masking out the parts of the rendered ground which I didn't want to show and overlaying a gravel texture on the ground to give it detail.
Here you will see my final adjusted background image and model
Step 9: Adding the passes
I then added the dust pass and metal/rust pass over the top and fully masked out the layers so I could paint into the masks to reveal the dirt, metal and rust where I wanted it to show. I like to use dirt and grunge brushes for this and stamp in where the dust and metal/rust will show.
These images show an example of painting in the mask to reveal the dust and the final dust pass and metal/rust passes on the vehicle
Step 10: Final touches
Once all the dust and metal/rust is revealed and I am happy with it, I painted some flying dust clouds over the scene using a cloud brush. The last step is to do some layer adjustments. I did a hue and saturation adjustment to play with the colors; a brightness and contrast adjustment to make the brights and darks stand out a little more; and I always like to put a photo filter over everything to make sure all the different renders and photos used share a similar color tint and feel like they belong together.
When rendering in mental ray with the Physical Sun and Sky you get a base image with a sky background. The colors of the sky leave a slight tint on everything in your render as does the sky in the real world. If you want to add a different sky to your final image it is important to adjust your render to match the color of the sky in the photo, as the sky color drives all the other colors in your scene. One of the biggest things that make an image look too CG is if the sky doesn't match the renders.
Gurmukh Bhasin's website
Photo-resource sites: the blueprints
and blueprint box
Military vehicle resource link