Painting and texturing the 3D render tutorial

The object I had to illustrate for this image was quite complex, so I decided to build a simple 3D model of it to take care of the perspective and major shapes. I used Cinema 4D to build my geometry and give it some simple materials. It was rendered with global illumination and with an alpha channel. You may need to render out several alpha channels, depending on the complexity of your models. It looks pretty good, but it's a bit dark and it doesn't mesh so well with my painted background.

The first thing to do is to load the alpha channel as a layer mask. That's why alpha channels are so important when you do your 3D render.  Since the image came out a bit dark, you can add an adjustment layer. I also added a color pass layer on top that brightened it up quite a bit. Do whatever color and value adjustments you need in Photoshop to get the render to match your background. The rendered image will be a base layer that the adjustment and painted layers will use as a clipping mask.

Start painting the wood texture on the ties. I used my favorite pencil thin brush again. The ties already had a wood material applied in Cinema 4D, but it didn't look so great and it was going the wrong way. I did a bit of loose painting to suggest the rough wood grain. This layer was clipped along with others to the render layer, so it was using that mask, but it needs one on its own layer so that the painting doesn't cover up the rails. I rendered out an alpha channel channel of just the rails, then loaded its selection to use as a mask on the ties painting layer. Invert the mask so that the rails are black and the background white. Now the painting doesn't cover up the rails. Since renders can be very time consuming, you can turn off options like textures, lights, ambient occlusion, etc. when rendering alpha channels. This will save some time. 

Continue painting other details on additional layers. Multiply mode works well for dark areas. For the solar panel, work on a new layer. Start with a simple shape with lines drawn across it. The final look is achieved with Layer Styles. The bright glare comes from a gradient overlay, using a light blue in Color Dodge blending mode. Using this mode with light colors is a great way to simulate luminosity or strong, specular highlights. 

The final details will be painted in on the top layer. Use small brushes for the most part to hint at the details that aren't in the model. You can also try some scatter brushes to suggest dirt or rust. This is how the painting looks on its own layer. You can see that it relies heavily on the 3D model underneath. The train and tracks also need some shadowing to tie them into the background. Right now, they look like they are floating above everything, so they need some shadows. For outdoor sunlit scenes, I like to use PMS 296 to give the shadows a bit of blue. Shadows are rarely pure black; warm lights will cast cool shadows. Since they are outside of the alpha channel, put them down in the ground group above the ballast layers. Use Multiply mode, which will darken the underlying layers, while letting some of their details show through. Remember that shadows are darker and sharper the closer they are to the object casting them and get lighter and softer as the distance increases. 

Now you can see how it is all coming together. The crisp, clean lines of the 3D model are softened and made to look less perfect and uniform. There is a bit of looseness which matches the look of the background. As a final step, note that all the layers of the train and ties are inside a layer group. Add a mask to the group and you can paint out the edges of the ties so that it looks like they are sunk into the ballast. Doing this on the group's mask keeps the mask made from the rendered alpha channel pristine. I try to avoid painting on these masks. What is really cool is that layers like the ties painting are being affected by three masks at the same time: the opacity of the bottom layer they are clipped to, their own layer masks, and the mask of the entire group. This allows for a great deal of flexibility.

In the next tutorial, we will continue with the details and finish off the train and rails.