Using alpha Channels within Illusion

The purpose of this tutorial is to aquaint Illusion users with the alpha channel features of Illusion and paint programs that allow one to mask out unwanted parts of emitters. The example I chose is an animation of of an M.C. Escher art print entitled 'waterfall'.

The resulting animation can be download from Impulse's Illusion movie page.

As you can see below, the Escher print is a rather unique 2D drawing of an impossible 3D scene. What I wanted to do was give it some 'life' and depth by animating the water and the waterwheel.

I first tried blockers to mask out parts of the emitters but they became too complex and caused a lot of crashes. The answer (suggested by Alan) was to use alpha channels. So, never having used them before, I decided this was a good time to start and jumped in head first!

The first step, of course, was to scan the picture from a book I have and import it into Adobe Photoshop. Any paint program in which you can create and work with alpha channels will do. The image I worked with was arbitrarily sized to 640 x 780, but to save space I have reduced it here.

Then I created an alpha channel for it and carefully blacked out the aeas where I wanted the water (Illusion emitters) and the waterwheel (an Imagine object) to show. Alpha masks are black where you want total transparency and white where you want no transparency. Shades of grey will give you vairations in between, something you can't acheive with blockers..

The strange image below is the alpha channel layer of the above picture. I made the waterfall part grey so the part of the building would faintly show through as in the original picture..

The next step was to contruct a 3D wooden waterwheel in Imagine. Nothing too fancy here since it's not seen close up.

The waterwheel by itself (except for a couple of light sources) was set up in the Imagine stage editor with the original picture as the background. The size, and position with respect to the cameral were adjusted until my waterwheel was superimposed exactly over the wheel in the picture.

Next, a 180 frame animation was created at the same 640 x 780 size. The wheel was rotated 360 deg. using the Rotate FX. Before rendering 'Generate alpha channel Image" was checked in the render settings dialogue. Below is what a frame from the animation looks like.

Imagine simply replaces any background picture with the background colors set in the globals actor dialogue. I set them to black for total transparency.

Now it's time for Illusion!

I couldn't find a suitable ready made water emitter for the stream so I got to work creating one that used the 'foop' image for its particles. The result was a line emitter with a 90deg emission angle. The shimmering effect was obtained by using the repeat color function set to maximum and and a 2 'cycle' color gradient. The velocity, number and visibility had to be adjusted to fit this particular use.

Here is the Illusion screen showing my stream emitter, the placement and direction of all the emitters, a blocker and the use of 3 layers.

The placement of the emitters and the alignment of their points is such that the particles flow parallel to the water channels.

The one blocker is required to prevent the particle flow of one of the emiitters from showing up in the waterfall part. This necesitated a third layer.

Now here's an explanation of the layers along with an enlargement of that part of the screen.

The bottom layer (Layer 2) contains 2 emitters and the blocker and uses the waterwheel animation from Imagine(with alpha channel) as the background.

The middle layer (Layer 0) contains the balance of the emitters. Note that layers are implemented in the order shown in the layer window and not by the numbers in their names. It's easy to shuffle layers up and down by right clicking in the highlighted (pink) area. The background color of the work area (seen here) determines the color that ultimately shows through the water emitters. Since we are dealing with a greyscale image here, a mid grey color was chosen.

The top layer (Layer 1) contains only the Escher waterfall background picture.

Now, one last step before we press that red animate button. We need to tell Illusion to use transparency in Layer 1 and Layer 2 above.

Right click on the little background picture or the 'T' to get the layer menu and then click on 'transparency' to get the menu below.

We select 'use existing image transparency' since our images have an alpha channel.

Here's what the Layer 1 transparency looks like.....

...and here's what the Layer 2 transparency looks like.

All done! Click on that animate button!

Paul McDonell