Rendering Background Images

Rendering background images in Blender is far from as easy as it should be and it is also far from obvious how to do it. Firstly, make sure your background image is the same size as that of your intended final render, otherwise it will be distorted. Select the Render icon in the Properties panel and set the render size accordingly, as in Figure 1.

Figure 1

Next, select the World icon in the Properties panel and then the Textures icon. This tells Blender that we are going to be adding a texture to the World.  See Figure 2.

Figure 2

Then click on the New button to add a new texure and Blender will create a new Clouds texture, as in Figure 3.

Figure 3

Now change the texture type from Clouds to Image or Movie. Click the Open button and navigate to your background image and load it. See Figure 4.

Figure 4

Change the texture orientation to World, as in Figure 5.

Figure 5

Scroll down to the Influence section and de-select Blend and select Horizon as in Figure 6.

Figure 6

Go back to the World panel and choose Paper Sky, so that Blender scales the image to the correct proportions. See Figure 7. Render.

Figure 7

Figure 8 shows a sample render.

Figure 8

Why it has to be such a long-winded process to get a simple background image into a render, I don't know, but there we have it.

Now let's look at a more flexible method of using background images. This involves using Blender's compositor, so open up the compositor and make sure Use Nodes and Backdrop are ticked, as in Figure 9. Now render your scene again, but without the background image. Back in the compositor add the nodes shown in Figure 9. The Render Layers node will hold your last rendered image automatically, which is a nice feature. The Image node will need to have your background image loaded into it. When you first add an Image node, it has an Open button at the bottom of it. Use this to find your background image and load it. The AlphaOver node ensures that the alpha channel from the Render Layers node is correctly applied to the final composite, to give a nice clean result. Without this node, the alpha channel is not properly aligned and the result can look quite nasty, with jagged edges around the foreground elements of the scene.

Figure 9

Figure 10 shows a Translate node now in the composite. This allows us to move the background image in any direction we like. In this example I've moved it up on the Y axis to show more of the clouds, which is what I wanted in the first place. Using the compositor is a far more flexible alternative to rendering the background image in the scene. If you decide later that you need a different background, it's simply a matter of loading it into the compositor, without the need to re-render anything.

Figure 10