Exercise 4: A flame that grows and fades

Purpose. To make a particle change during its life: grow, fade, and shift colour.

Every value entered so far has stayed the same from the moment a particle was made until it vanished. A flame does not work that way. Each puff starts small and bright, swells, darkens, and thins out to nothing. A value that changes over a particle’s life is drawn as a graph.

Before starting

The workspace holds one new Part with Shape Ball, Size 1, 1, 1 and Material Neon. Set these in Studio’s own Properties window.
Auto-emit from Exercise 3 is off.

Procedure

Steps 1 to 4 repeat what Exercises 1 and 2 taught. The new work starts at step 5.

Step	Control	Action	Result
1	The Part	Select, then press Transform	The Part turns invisible. The HUD and the Properties panel open.
2	Properties ▸ Motion ▸ Speed	Enter `6`	No change yet. Nothing is being emitted.
3	Properties ▸ Motion ▸ SpreadAngle	Enter `10`	No change yet.
4	Properties ▸ Emission ▸ Enabled	Switch on	Ten balls a second rise in a narrow column and vanish after one second, all the same size and colour.
5	Properties ▸ Appearance ▸ Transparency ▸ ⋯	Press	The graph editor opens, titled Graph - Transparency. It shows a level line at 0 with a keypoint at each end.
6	Graph editor ▸ right-hand keypoint	Drag up to 1	The line now climbs from 0 to 1. In the viewport each ball fades as it rises.
7	Graph editor ▸ Apply	Press	The editor closes. The Transparency box reads `<NumberSequence>`.
8	Properties ▸ Appearance ▸ Size ▸ X, Y, Z	Click each until all three are lit	The box beside them reads 1.
9	Properties ▸ Appearance ▸ Size ▸ ⋯	Press	The graph editor opens, titled Graph - Size (X+Y+Z), with a level line at 1.
10	Graph editor ▸ left-hand keypoint	Double-click, enter `0.4` for Value	The line starts at 0.4. Each ball is made small.
11	Graph editor ▸ right-hand keypoint	Double-click, enter `2.5` for Value	The line climbs from 0.4 to 2.5. Each ball swells as it rises.
12	Graph editor ▸ Env box	Enter `0.2`	A band appears around the line. Some balls now run larger than the line and some smaller.
13	Graph editor ▸ Apply	Press	The editor closes.
14	Properties ▸ Appearance ▸ Color ▸ ⋯	Press	The colour editor opens, titled Color - Color. It shows a bar of one colour with a marker under each end.
15	Colour editor ▸ left-hand marker	Click to select, click again, enter Red `255`, Green `240`, Blue `170`, press OK	The left end of the bar is pale yellow.
16	Colour editor ▸ right-hand marker	Click to select, click again, enter Red `120`, Green `20`, Blue `0`, press OK	The bar runs from pale yellow to dark red.
17	Colour editor ▸ the bar, at its middle	Click	A third marker appears there, already selected.
18	Colour editor ▸ the new marker	Click, enter Red `255`, Green `120`, Blue `0`, press OK	The bar runs yellow, orange, dark red.
19	Colour editor ▸ Apply	Press	The editor closes. Each ball starts small and pale yellow, swells through orange, and fades out dark red.
20	Properties ▸ Emission ▸ Enabled	Switch off	The flame stops.

What the graphs did

A graph is read from left to right. The left edge is the moment a particle is made. The right edge is the moment it vanishes. The height of the line at any point is the value the particle has at that age.

The left-to-right scale is a share of the particle’s life, from 0 to 1. It is not measured in seconds. The same graph therefore fits a particle that lasts half a second and one that lasts ten. The long-lived particle plays the same shape more slowly. See The 0 to 1 scale.

Transparency runs from 0, fully visible, to 1, invisible.

Size is measured in studs. It is the particle’s actual size, not a multiple of the Part’s size. Transform set the three Size graphs to the size the Part had, which is why they read 1 here. The X, Y and Z buttons choose which of the three graphs the box and the ⋯ button act on. With all three lit, one graph is written to all three, and the ball stays round.

Env gives a graph a random spread. Each particle is made a little above or below the line and stays on that side for its whole life. With a spread of 0.2, one ball may run from 0.6 to 2.7 and the next from 0.3 to 2.4. A colour graph has no spread.

Between keypoints a graph runs in a straight line. The graph editor can also bend the line smoothly. That control and the others, among them presets and copy and paste, are described in The graph editor.

If the result differs

Symptom	Cause	Action
⋯ on the Size row does nothing	None of X, Y, Z is lit.	Repeat step 8.
The ball stretches into an egg	Only one or two of the Size buttons were lit at step 9.	Light all three and repeat steps 9 to 13.
The end keypoints will not move sideways	The first and last keypoints are fixed at the start and end of life. They move up and down only.	Drag up or down.
The colour picker does not open at step 15	The marker was clicked once. The first click selects it.	Click the selected marker again.
The drawn graph is gone and the box shows a number	A number was typed into the box.	Press Ctrl+Z, or draw the graph again.

Variations

Change	Result
Click the middle of the Transparency graph and drag the new keypoint down to 0	Balls stay solid for the first half of their life and fade in the second.
Speed `4, 8`	Each ball takes one speed of its own between 4 and 8. Two numbers in a graph’s box give a level line with a random spread.
Lifetime `0.8, 1.6`	Longer-lived balls rise higher and play the same graphs more slowly.
Size with only Y lit, a line from 1 to 4	Balls stretch upward into streaks.
Colour editor ▸ Invert	The gradient is reversed: dark red first, pale yellow last.

Log


New in this exercise	Graphs. The graph editor: keypoints, Env, Apply. The X, Y and Z buttons. The colour editor.
Done before	Transform (Exercise 1). Speed (Exercise 1). SpreadAngle (Exercise 2). Enabled (Exercises 1 and 2).
Carried forward	Keep this emitter. Exercise 10 uses it again.