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Bryce users have asked how clouds can be animated in a loop that repeats seamlessly, without a “pop” at the join. Here's a method I used back when Bryce 4 was new. Bryce 5 is used in this example.
Here is a Bryce 5 scene at startup, with a small-sized canvas (to save time), and a render from the camera view.
I create or load my scene. This is a single terrain and material from the standard Bryce library. I lowered the camera and tilted it slightly upwards.
I open the Sky Lab and turn off the cloud textures and the “Link Sun To View” button. (I may need to reposition my sun once more.) Now my sky is cloudless with only the sun glow.
I added to the scene a single cloud plane and gave it a cloudy material from the standard Bryce library, “Wispy Afternoon” in this example. Save the Bryce file.
This is where I position my plane, at XYZ= 0, 40, 0. These nice round numbers will be important in a minute, because they'll need to be duplicated. Its name is now “CloudPlane 1”.
I open the Materials Lab to edit the cloudy texture, because the default scale makes the clouds too big to suit me. Increasing the scale in XYZ from 25% to 50% shrinks the clouds nicely. (Seems backwards to me. Oh, well.)
If you use a different cloudy material, make sure the texture is not mapped using any of the “World” spaces. I prefer “Object Space”.
Now I've opened the Deep Texture editor for my cloud texture source to show you something. Notice that the noise source for my clouds is a 3D noise, and that the Z frequency is about the same as the X and Y frequencies. This will become important when I add motion. If you use cloud textures made with 2D or 1D noises, it's good to change them to 3D and adjust the Z frequency to a number close to X and Y.
Here's my scene now, good enough to show the effect. Time to give it some motion.
I make sure that “Auto Key” is active and add a keyframe at exactly 2 seconds. In this example, my loop will be 2 seconds long, so I also set “Play Mode” to “Repeat”. Save the file.
Duplicating my cloud plane (Ctrl+D in Windows) produces “CloudPlane 2” in the same centered position. Now I'm going to move the cloud planes and change their keyframe positions. CloudPlane 1 will have its keyframe repositioned at 2 seconds, and CloudPlane 2 will be repositioned at 0 seconds. Their other keyframes should remain matched, and I'll leave them untouched. Here goes.
The plan is that both planes should move at the same speed and direction, and the position of 1 at the start is exactly matched by 2 at the end, so I'll make the changes very simple. By entering the following numbers into their attribute windows, these will be the position values at 0 and 2 seconds…
X = 0 to 80
Y = 40 to 35
Z = 0 to 0
X = -80 to 0
Y = 45 to 40
Z = 0 to 0
(Both planes are traveling westward and descending slightly.)
I can preview my motion if I shorten the workspace in Bryce's timeline by one frame. In my example the workspace extends from 0 to 1:14.
Now I'm going to keyframe the plane materials in a manner similar to their position values. In my example the transparency value is 0% (other textures may produce different values), and I add keyframes at 0 and 2 seconds for each plane. Then I change the transparency values. CloudPlane 1 is set to 100% at 2 seconds, and CloudPlane 2 is set to 100% at 0 seconds.
Here's where the 3D nature of the noise sources makes the difference. I'm going to keyframe the textures on the planes as they dissolve and fly. This will add a little turbulence to the cloudy motion. Again, the values will have matched keyframes and speeds.
CloudPlane 1, texture position Y = 100 to 116
CloudPlane 2, texture position Y = 84 to 100
Save the file.
A preview of the loop shows that the motion is starting to look smoother. One plane of clouds melts away as its counterpart melts in. The motion no longer has a “pop”, but it does have a “bounce” in density at the ends, and the density gets very sparse in the middle. Let's make some adjustments.
I select both cloud planes and open the Advanced Motion Lab. Expanding the material channels, I select the Transparency channel for one plane and assign the “ease-in, ease-out” motion curve to it. Then I do the exact same thing for the other plane's Transparency channel. This will soften the “bounce”.
Now I select CloudPlane 1 and move my timeline slider to the halfway point, 1 second in this example. That's where the clouds become thinnest. I'm going to thicken them.
In the Materials Lab, I change the transparency at the 1 second point, and a keyframe will be added there automatically. I reduce the transparency, but not as low as at the starting keyframe (which was 0% in my example). I'll use a roughly 10% value. Then I select CloudPlane 2 and reduce its transparency at 1 second in the same way. Save the file.
The basic scene is ready to render. It will not “pop”, but will resemble time-lapse footage of windy weather.
There are many ways to improve the result, by experimenting with different rates of change, different durations in time, different texture sources, additional pairs of cloud planes. These can improve on the remaining shortcomings, like the still visible transition between cloud patterns, and the non-uniform nature of linear motion in Bryce. Have fun.
Praise the Lord!