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- We are going to continue
showcasing some of the

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features of OpenGL, and in this segment

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we are going to talk about animation.

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First, let me show you
a demo of the program.

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In this case, I will set
the DEMO is equal to 3.

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We now can see that we have a teapot,

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and if I hit the P key,
the teapot animates.

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While it is animating, I
can zoom in and zoom out

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as before, and I can also stop
and restart the animation.

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Let's look at the code for this purpose.

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We want to put a teapot in
the middle that animates.

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The first line of code is
pushMatrix of modelview.

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Then you multiply the
modelview matrix on the right,

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as in old OpenGL, by a
translation corresponding

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to the teapotloc in the X coordinates,

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no translation in Y and Z.

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That's what causes the teapot to animate

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from the left to the right.

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We'll just need to increase
the teapotloc variable.

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Notice again that this translation command

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where the original matrix
input is the identity,

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that ensures that glm::translate

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returns a translation matrix.

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Alright, so the next two transforms
setup and center the teapot.

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And again, the transforms right multiply

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the modelview matrix, which
is the top of the stack.

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So these transforms, which
are multiplied by translation,

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by Z is equal to 0.1, just
set up and center the teapot.

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There's also rotations so
that the teapot appears

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in the correct orientation.

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Notice that all of these
transforms are right-multiplying

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the top of the stack, so in
fact the last transform in code

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will be the first transform applied.

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So glm::scale sets the scale
for the teapot appropriately,

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then you rotate the teapot
into the correct location,

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then you translate it so
it appears on the table,

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and finally you do what we're interested in,

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which is the animation part of it,

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which is translation by teapotloc.

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Notice here the glUniformMatrix4fv command,

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which sets modelviewPos
to the correct value

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for the modelview matrix,
and this a uniform matrix,

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and once you've done all this,
you pop the modelview matrix

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from the stack so the rest of
the geometry is not affected

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by what you've been doing for the teapot.

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And this a very common idiom,
that a particular object

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is defined between push and
pops for the modelview matrix.

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We did the same thing for
each of the four pillars in drawing.

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Here is my animation routine.

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So the void animation
command simply states that

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teapotloc is equal to
teapotloc plus 0.005.

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That means at each step,
it slightly increments

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the location of the
teapot, which is reported

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for the animation and if the
teapot goes off the right edge,

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you simply bring it back to the left edge.

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glutPostRedisplay() just
redisplays the scene

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and tells OpenGL to call
the display function.

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drawteapot() in geometry.h

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binds the vertex array for the teapot,

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draws the elements corresponding
to the teapot element

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in the scenes and unbinds the array.

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The keyboard callback is where p pauses

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and restarts the animation.

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Notice that I have this animate variable,

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which is originally zero,

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and then I can toggle it
to 1 and back to 0.

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In the keyboard, I have
escape to quit as before,

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but now I added a new case for p

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to pause and restart the animation.

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It toggles the animate
variable; and notice this code.

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If the animate variable is on,

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it sets glutIdleFunct to the animation.

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So even if no keys or
nothing has been pressed,

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glut will idly call
the animation function,

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which will move the teapot.

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If animate is off, then the
glutIdleFunc() is null.

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It does nothing.