You've finally finished the rendering, but the background doesn't look right and needs to be replaced. Use of the alpha channel is the way to go.
This tutorial describes the use of the alpha channel in AccuRender.
An Alpha Channel represents transparency information about an image, on a per pixel basis. This can be very useful with the renderings created with AccuRender.
The following is how I’ve come to use the Alpha Channels of TIFF files which have been generated from a rendering in AccuRender. This may or may not apply to your use; therefore, please regard all of this as a general discussion only, and in no way infer any particular expertise of the author (ergo ME!).
Creating The Alpha Channel With AccuRender
To create an Alpha Channel with AccuRender, in the “Environment” settings dialogue, select “Alpha”, as shown below:
Once the rendering is complete, you need to save your image in a file format that supports the Alpha Channel. My preference is to save it as a TIFF file (Tagged Image File Format) as this is a lossless type image.
How To Isolate The Rendered Image Using The Alpha Channel
Using Adobe PhotoShop, open the TIFF file which you’ve created. Following is the sample that I’ll use for this demonstration:
Even though it isn’t obvious at this stage, this TIFF file contains an Alpha Channel which consists of all of the transparency information; that is, all of what is not the image that you rendered.
You can use this Alpha Channel as a mask to separate the actual image from the background in order that you can then insert other backgrounds, other images, text, etc.
To do this, proceed as follows.
On the menu bar in PhotoShop, click on “Select” and then “Load Selection”, as shown below:
You will next be presented with the following dialogue box:
The default shown above is fine, so click “OK”. Next, click on “Layer”, then “New”, then “Layer Via Copy”, as shown below:
Alternately, you can use the shortcut keystrokes to do this by holding down your <CTRL> key and simultaneously depressing the letter “J”. You will now have a new layer which contains ONLY the image; the background is not included.
To evidence this, make your new layer active and then turn off the visibility of the Background layer. You will see that only the image remains:
No matter how meticulously you might try to create this by silhouetting, you could never be as precise because the Alpha Channel is done on a pixel-by-pixel basis – simply put, it’s dead on the money!
Creating An Alpha Channel With Photoshop
The following is the way that I have come to create an alpha channel using Photoshop so that you can then mask out the background when you apply it in AccuRender (whether as a material definition or as a decal). It's not difficult to do, but it's also not so easy to figure out if you'd never done it this way before.
Begin by creating a new PhotoShop file and be sure to set it as "RGB" (not grayscale), even if your image doesn't need color. Next, create whatever you're going to create in PhotoShop. Save it as a native PSD file.
Click on the "Channels" palette, and from there, click the right-pointing triangle you see shown below. Next, click on "New Channel" like you see here:
It will default to the following, which is fine - accept it:
You will now notice that it has turned off each of the RGB channels (why this is, I have no idea). Turn all three of them back on. You will now see something like this:
Now click back on the "Layers" palette and make your selection using the ordinary way that you're used to.
In the example above, I simply held down the <CTRL> key and depressed the layer's name which selected everything "active" on that layer.
Now press the <DELETE> key, and you're done: Note that it deleted that selection from the alpha channel; it did not delete anything from the layer itself.
You will need to save the file out in a format that supports an alpha channel, such as a TIFF file, for example.
This completes the tutorial.
rev 03.20.2005 :: For more information visit http://www.accustudio.com/
rev 08.10.2005 :: Formatted for AccuStudio by Kevin Lockwood