Hands-on MAX: Compositing Reflections of a Mirror with Another Image

This series of exercises will give you practice with the concepts and procedures of compositing in MAX. We are going to composite an image into the mirror of our bathroom scene from the last chapter. We are going to deselect the mirror from the alpha channel and then select just the reflections in the mirror to add back to the alpha channel. This way, when we composite our render over our image file, the image will show through the mirror, with the reflections overlaid on top.

Assigning a Matte Material

First we need to change the mirror material into a matte, to give it a zero value in the alpha channel while still blocking the wall behind it.

1.  Open the file waterburst_4.max from the last chapter, or from the CD under Chapter 11, and save it as alpha_mirror.max.

2.  Using Select By Name, select the Mirror object. Open the Material Editor. Select the first sample slot, called Mirror. Click the Type button (the one that says Standard directly under the sample slots and toolbar). This will bring up the Material/Map Browser. Choose Matte/Shadow and click OK to leave the browser.

3.  Uncheck Opaque Alpha under the Matte/Shadow Basic Parameters. (We want our mirror to be selected as transparent, not opaque, in alpha channel.)

4.  Before we go any further, let’s just pretend that we’re not sure we’ve chosen the right setting to get the alpha channel we want. Click the Render Scene button, check Single Frame, make sure Save File is not checked, and click Render. The reflections in the mirror have disappeared. But let’s check the alpha channel.

	5. Click the Alpha Channel button in the Virtual Frame Buffer. Your alpha channel should look like Figure 12.10. The mirror is now black (that is, transparent), so we can composite another image there.

FIGURE 12.10  The alpha channel of the mirror

Capturing the Reflections with a Raytrace Map

We still want the reflections in the mirror, partially transparent over the image we will put in the mirror. To do that, we need to make a duplicate mirror object.

1.  With the Mirror object still selected, choose Clone from the Edit menu. In the Clone Options dialog box, check Copy and name the new object Mirror-reflections.

2.  We’re going to put a raytrace reflection map in the diffuse color slot to catch the color of the reflections. With the Mirror-reflections object selected, select a new sample slot in the Material Editor. (If you drag the line between the sample slots, you can scroll to new material slots.) Name the new material Mirror-reflection and click the Assign to Selection button. Click the Diffuse Map button next to the color swatch labeled Diffuse. Choose Raytrace from the Material/Map browser and click OK. In Trace Mode in the Raytracer Parameters rollout, check Reflection. Click the Global Parameters button, check On for Fast Adapative Antialiaser, and click Close.

	3. Now we need to put an instance of this map in its opacity slot so that only the reflections will render. Click the Go to Parent button and scroll down to the Maps rollout. Drag the Raytrace map from the Diffuse Color map button to the Opacity map button. Choose Instance from the dialog box and click OK.

4.  Right now the matte Mirror object is in the same place as the Mirror-reflections object, blocking the reflections as well as the wall. We need to move it back just a little. Close the Material Editor and select the Mirror object, using Select by Name. With the camera viewport active, click the Move tool and then right-click the Move tool. Type 0.01“ in the X value of Offset:World and press Enter. Press Shift+E to Render Last. Your image should look like the one in Figure 12.11, with the reflections now showing in the mirror.

5.  Click the Alpha Channel button. Your alpha channel should now look Fig-ure 12.12. Note that the reflections in the mirror are shades of gray, meaning that they will be partially transparent when composited.

FIGURE 12.11  Captured reflections, after moving the matte Mirror

FIGURE 12.12  The new alpha channel

Rendering a 32-bit Targa Sequence

This time we’re going to render to a series of Targa files with alpha channels.

1.  With the same scene file open that you have been working on, click the Render Scene button. Check Active Time Segment and 320 x 240.

2.  Click the Files button. Choose the directory where you will be saving your files. Choose Targa Image File from the Save As type drop-down list. Type alpha_mirror in the File name field. Click the Save button.

3.  In the Targa Image Control dialog box, check 32 Bits-Per-Pixel, Compress, and Pre-Multiplied Alpha. Uncheck Alpha Split. Click OK.

	NOTE It is the 32 bits per pixel that tells you that you’re saving the alpha channel: 8 bits for each of the red, green, and blue values of the pixel, and 8 bits for the alpha channel value. If you saved the same scene to 24-bit Targas, you would lose your alpha channel information.

4.  Double-check that you are rendering Camera01 and that Save File is checked. You are ready to render.

	NOTE If you don’t want to wait through the whole rendering process, the Targa files are available on the CD, in the \Ch12\Targas directory. You should copy them to your hard drive, so that it is easy for MAX to make the image file list (IFL) file.

5.  Save your scene file before you render. If you are going to do the render yourself, then, um, click Render.

© 2000, Frol (selection, edition, publication)