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In this tutorial, we'll use different types of Booleans to create a broken glass effect.
First of all create a sphere. Enlarge it and move it up off the ground. Click on the A button and select positive.
Duplicate the sphere (Ctrl - D) and reduce the duplicate in size slightly. The difference in size is the thickness of the glass. Click on the A button and select negative. Select both spheres and group them together (G). Click on the A button and select positive for the group. You now have a hollow sphere although you can't tell by looking at it.
Create a cube. Enlarge it so that it completely encloses the hollow sphere when viewed from the top. Reduce the height of the cube and move it vertically to the position shown. I'm using the view from the right in Figure 2. Click on the A button and select negative. Group the cube and the hollow sphere together. Click on the A button and select positive.
Create a rock, not just any rock, try to get one that's roundish rather than one of the more rectangular ones. Delete and recreate until you find a suitable one. Raise the rock and position it to intersect the glass bowl rim. Use side, front and top views to help in the positioning.
The rock should have the same glass material applied as the bowl. With the rock selected, click on the A button and select negative. Group the glass rock with the bowl group. The resultant render is shown in Figure 5. Attach a glass cylinder for a stem and make a base from a flattened sphere with the bottom cut off by subtracting a cube. There you have your broken glass.
But wait - there's more and this is the really neat bit. Move the whole group (rock and bowl) to one side. While it's selected press Ctrl -D to duplicate the lot. (Cmd - D on the Mac). Move the duplicate to the left, to clear the original group. Click on U to ungroup the rock from the bowl. With the rock selected, click on the A button and select Intersect. Regroup the rock and the bowl. There is the missing piece.
By using the shift key and rotating 90 degrees or 270 depending which way you go and then lowering the group you can position the broken piece on the ground plane where it fell. Use a side view and zoom to position the curve of the outer sphere on the plane. A subsequent render is shown in Figure 8.
But wait there's more. This broken piece of glass is an exact fit for the gap in the glass bowl. So if you just want a cracked glass just select both groups and hit the centre align button. The piece will be fitted into the gap.
You must do this before the previous rotation or it won't work. Notice that the crack is visible in the projected patch of light. Play with the lighting position to get the crack to sparkle.