Having the material ID channel has been an immense help in compositing images in post-process. This is a tool that allows you to mask out a material and replace it with a suitable texture. I thought I would share my steps in making this image using the material ID and Alpha channel.
Step 1. select material ID color. In the material editor, select the material you want to mask, and pick a color other than black. Also, use a color for the diffuse layer that is low saturation, this will be important later.
Step 3. When you render your image, save the image as a “.png”, and you will have three files. Your rgb color map, an alpha map (which is also the transparency in your first png), and a material_id map. (color and material ID shown below:)
Step 4. Open the color and material ID images into your image editor (one that can manipulate and overlap layers or levels) I am using Photoshop, so the rest of this is referencing PS.
Step 6. Arrange the three images so that material ID is on top, then color image is next, and the background is at the bottom of the stack. Make a copy of your color image.
Step 7. Make a selection your material ID color on the first layer by using the “magic wand” tool in PS, set to a tolerance of 5. Then choose the color layer in your layer window and run the following command: Layer -> Layer Mask -> Hide selection.
Step 8. Make a selection of the black color on the material ID layer. Choose the copy of the color layer in your layer window, then run the following command: Layer -> Layer Mask -> Reveal selection. Next turn off (or delete) your material ID layer. Change the transparency of the copied color layer to “multiply” – this will cover the background with the shadow pattern from the render. If you have a color that is low saturation, the shadows should look fine. If you have a strong saturation, you may need to reduce the saturation on this layer so the colors are more normal. Adjust transparency as needed (mine is at 74%)
Step 9. Create a new layer and airbrush in some shadows under the bushes.