Blending Two Materials in Photoshop
© 2009 Robin Wood
This is a short tutorial showing one very easy way to blend two materials in Photoshop. For instance, blending a wireframe with a render.
In this example, I’m going to use a goldfish that I made with NewTek’s LIghtWave 8. I’ve rendered it with Luxology’s modo, but you can use anything at all; even things that were never 3D. It’ll work the same way, no matter what it is.
First, we need to have the two images we want to blend. In this case, two renders of our object; one as wireframe, and one fully rendered.
How you do this will depend on your 3D program.
If possible, of course, just render it both ways.
In modo 302, which I’m using here, there’s no simple way to render a wireframe, so I took a screen shot, and I”ll be using that instead.
Open both images in Photoshop, as I’ve done. This is Photoshop CS4 Extended, but it’ll work the same way in any version, even if the interface looks quite different.
Since these two aren’t exactly the same size, I’m going to resize the wireframe, so that it matches the render.
I can see the size I need here, below the image. (I set that up with this black flippy, in CS4, by using Show .. Document Dimensions. If you have a different version of Photoshop, you’ll set it up a different way, but it’s always in this general area somewhere.)
So, go to Image > Image Size, and we’ll set it to Constrain Proportions, since I know it’s already a square, and just change the width to 512 pixels. The Height will change to match it.
Then, hold down the Shift key, and just drag the wireframe into the same window as the fully shaded version.
Since they are exactly the same size, holding down Shift will constrain the position, so the wireframe will be precisely where it was in the original document.
Now, if these had both been renders, they would automatically match each other perfectly.
Since they’re not, I have to make sure they match manually.
The best way to do that is to set the Blend Mode of the top one to Difference. That makes it more obvious when they fit, and when they don’t.
I’m going to zoom in a bit, so I can see this better. I’m using the keyboard shortcut - command spacebar, that’s control spacebar on a PC, to zoom in.
Then just use the arrow keys to nudge it a little bit. 
That looks good. Check the other end; use the hand tool. Spacebar to get the hand tool. And that looks pretty good. Okay, I think that’s good.
If yours don’t match, then you might be able to use the arrow keys to nudge the top one into position, or you might need to Transform it, using Command T, that’s ctrl T on a PC, so it’ll fit.
Change the Blend Mode back to Normal
We’ll use the Alpha that was made durning rendering to start the masking process that will make the blend.
Check in the Channels palette, and you can see that we have an alpha ready and waiting.
Back to Layers.
So go to Select > Load Selection, choose the Alpha, and click OK.
Then click the New Mask button, at the bottom of the Layer panel, to make the mask.
Like that.
As you can see, the Mask looks exactly like the Alpha Channel did; and it’s masking out everything but the object.
Make sure the Mask is selected. If it is, it’ll have this outline. (Different versions of Photoshop have different indicators.) Set the color in the Color Swatch over here to Black and White by tapping the D key for “Default Colors”.
Now I’ll get the Brush tool. Check here in the Brushes palette. Make sure it’s soft, here in Brush Tip Shape, good. 
Make sure that Other Dynamics is checked, then click on that field to make sure the Opacity is controlled by Pen Pressure, since I’m using a tablet, and just start to paint with black on the mask, to hide more of the wireframe.
I can increase and decrease the brush size with the bracket keys, or by using the slider on my graphics tablet. (Or, since I’m in CS4, I can change the brush size interactively by holding Control Option, (that’s ctrl Right Mouse Button on a PC) touching the brush to the image, and dragging right and left. I can also change the softness interactively by holding Control Option Command on a Mac, ctrl alt Right Mouse Button on a PC, and dragging.
Now I’ll just paint out what I don’t want. Or, if I remove too much, I’ll paint it back in with White.     
You can tap X to switch the foreground and background colors.
Like this.
And that’s really all there is to it!
This is Robin Wood, and I hope you find this helpful.