Is it possible to make something disappear periodically? Maybe by making
one dimension (radius or linear_extrude etc.) of a shape alternate between
zero and a positive value at regular intervals?

The problem: the cams in the animation go 180° at half-speed then jump
180° so they effectively catch up to end up going at full speed. If they
could be made to disappear every second pass, and a second cam in each case
pops up and continues where the first disappears, until after 180° it is
replaced by the first cam, that would give the illusion of continuous
half-speed motion.

I know the standard solution would be to double all the speeds. But there
is a further shaft with gears that already run at double speed, so they
would have to run at quadruple speed. Instead of at least 180 frames, it
would need 360 frames to avoid the artefact of gears appearing to run
backwards or just looking fuzzy. That gives a rather large file and, with
max. 50 fps, a slow animation.

$vpr=[ 170, 315, 45 ];

$vpt=[ 0, 0, 0 ];

$vpd=700;

module cams()

{

hull()

{

cylinder(r=19, h=5);

for ( i = [0, 20])

rotate([0, 0, i])

translate([14, 0, 0])

cylinder(r=10, h=5);

}

translate([0, 0, 12])

rotate([0, 0, 90])

hull()

{

cylinder(r=19, h=5);

for ( i = [0, 20])

rotate([0, 0, i])

translate([14, 0, 0])

cylinder(r=10, h=5);

}}

// Timing gear

rotate([0, 0, $t*360])

color(steel1)

linear_extrude(6, center=true)

gear(30, 24, 2);

// Timing gears

for ( i = [0:1:3])

rotate([0, 0, 90*i])

translate([72, 0, 0])

rotate([0, 0, 3i+4 - $t180])

color(steel1)

{

linear_extrude(6, center=true)

difference()

{

gear(60, 48, 2);

for ( i = [1:1:8])

rotate([0, 0, 360/8*i])

translate([30, 0])

circle(8);

circle(12);

}

translate([0, 0, -28])

difference()

{

union()

{

cylinder(r=16, h=32);

translate([0, 0, 0])

cams();

}

translate([0, 0, -0.5])

cylinder(r=12, h=33);

}

}

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Don't you just want to rotate the inner gear twice as far because it needs
to rotate twice for a full rotation of the cams? You don't need more
frames, it just needs to move twice the angle in each frame.

A moving “now you see it, now you don’t” difference() might be worth a try
(but not right now).

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You don't need to double the speed, you just need the gear to rotate twice
as far. You can make the animation last twice as long.

To make something disappear you could just wrap it with if($t < 0.5).

nop head wrote:

You don't need to double the speed, you just need the gear to rotate twice
as far. You can make the animation last twice as long.

Twice as far in twice the time sounds interesting! Can that be applied to
$t*180 to get full 360° rotation with half-speed?

To make something disappear you could just wrap it with if($t < 0.5).

I have got this far:

if($t > 0.5)

rotate([0, 0, $t*180])

translate([101, 0, 0])

color("red")

circle(50);

if($t < 0.5)

rotate([0, 0, $t*180])

translate([-99, 0, 0])

color("blue")

circle(50);

rotate([0, 0, $t*360])

color("black")

square([2, 160]);

The black line rotates at $t*360. The coloured circles go round at half
speed, but only do half a circle. How can they be made to do a full circle
at half-speed?

Or there is this:

rotate([0, 0, $t*180])

translate([101, 0, 0])

color("red")

circle(50);

rotate([0, 0, $t*180])

translate([-99, 0, 0])

color("blue")

circle(50);

rotate([0, 0, $t*360])

color("black")

square([2, 160]);

How could the two circles be made to disappear alternately, instead of
simultaneously, to give the impression of a single circle that changes
colour instead of two? Can if($t < 0.5) be advanced/retarded by 180°?

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To make the inner gear turn twice as far use $t * 720, to get two turns as
$t goes from 0 to 1.

To make the animation take twice as long, so that it rotates at the same
speed, double the number of steps, so each frame increases $t half as much
and you twice as many frames. Or you could halve FPS but then it might look
a bit jerky.

nop head wrote:

To make the inner gear turn twice as far use $t * 720, to get two turns as
$t goes from 0 to 1.

Yes, that would work, but no I don’t want to do that because the extra
shaft would then have to rotate at $t1440 instead of the present $t720.
The model I am working on has parts rotating at three different speeds:
camshafts rotating at half crankshaft speed and a synchronising shaft
(that’s what I will call it) rotating at double crankshaft speed.

To make the animation take twice as long, so that it rotates at the same
speed, double the number of steps, so each frame increases $t half as much
and you twice as many frames. Or you could halve FPS but then it might look
a bit jerky.

The animation would take twice as long by the clock on the wall, but the
$t clock would remain unimpressed.

Is there some other way to get either real or virtual full 360° rotation
with $t*180? I think I need a time machine. Must see what ebay has to offer.

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$t isn't a clock. It is just how far through the animation you are. It goes
from 0 to 1 in the number of steps you specify. That clock time that takes
will be the FPS / steps.