Tutorials · Particle Effects · Chapter 5

Curves and gradients

Fire over its life

1. Select Town, press + Add node and choose Particles. Rename it forge_fire, and set its Position to 114 and 280, the coals in the forge’s mouth.
2. Set Rate to 40, Lifetime to 0.9, Speed to 28, Spread to 0.5 and Gravity to -30. Direction stays 0, -1, straight up.
3. Set Start Color to 1, 1, 1, alpha 1, and End Color to 1, 1, 1, alpha 0. If the picker’s button at the top left reads U8, press it so it reads F, as in chapter 2. Both are white so that the gradient below decides the colour, and the alpha still fades to nothing.
4. Set Start Size to 10, End Size to 6, Capacity to 40, Sort Layer to 3 and Blend to Add. Set Texture to soft-dot.png.
5. Open Emission Shape, set Shape to Box, and Size to 32 and 4, the width of the coals. Leave Edge Only unticked.

1. Tick Color Over Life and press the arrow at its left. A bar opens, plain white: a gradient with no stops changes nothing. The bar runs from 0, birth, at its left end to 1, death, at its right.
2. Click the strip just above the bar, at its left end. A square appears: a colour stop, selected. Beside Step are its swatch and its place along the bar, at. Click at, type 0 and press Enter. Then click the swatch and set it to 1, 1, 1, white.
3. Click the strip about an eighth of the way along, set at to 0.12, and set the swatch to 1, 0.8, 0.25, a yellow. Add a stop a little before the middle, at 0.4, set to 1, 0.4, 0.08, an orange, and one at the right end, at 1, set to 0.35, 0.33, 0.32, the grey of smoke.
4. Now click the strip just below the bar. These are alpha stops, which are set apart from the colours: a stop here sets how solid the particle is. Beside Step are its alpha and its at. Add three: alpha 1 at 0, 0.9 at 0.7 and 0 at 1.
5. To move a stop by hand, drag it along the bar. Right-click a stop, or press Delete with it selected, to take it away.

Color Over Life

A gradient of colours and alphas across a particle’s life, from birth at 0 to death at 1. It is multiplied with the run from Start Color to End Color, not put in place of it, so with both of those set to white the gradient alone decides the colour. Colour stops take three numbers from 0 to 1; alpha stops take one. Here, four colour stops and three alpha stops, from white through yellow and orange to grey. Left unticked it does nothing: an empty gradient is white.

at

Where the selected stop sits along the bar, from 0 at its left end to 1 at its right: on Color Over Life, the moment in a particle’s life the stop’s colour or alpha is reached. Here the yellow is reached at 0.12 of the life and the orange at 0.4. A stop typed past its neighbour stops there.

Size Over Life

A curve that multiplies a particle’s size across its life: 1 leaves the size as Start Size and End Size run it, 0.5 halves it. Here it climbs from 0.6 at birth to 1 a third of the way up the life, 0.3, then falls to 0.4, so a flame swells as it takes hold and shrinks as it burns out. 0.4 to 1 suits a flame; a curve that reaches 0 makes the particle vanish at that point.

Velocity Over Life

A curve that multiplies how far a particle travels each step, and with it all of its speed, gravity’s share too. Here it falls from 1 at birth to 0.3 at death, so the flames slow as they die and stay near the coals. 0.3 to 1 suits a flame. A curve that reaches 0 stops the particle dead, wherever it is.

x and y

The selected point of a curve. x is where it sits along the canvas, from 0 at the left to 1 at the right: on an Over Life curve, how far through a particle’s life. y is the number the curve reads there, the one that multiplies. Here the size curve’s middle point is at x 0.3 with y 1. A point typed past its neighbour stops at it.

1. Tick Size Over Life and open it. Double-click the line about a third of the way along to add a point there, and set its x to 0.3. Its y is already 1.
2. Click the left point and set its y to 0.6. Click the right point and set its y to 0.4.
3. Tick Velocity Over Life and open it. Click the right point and set its y to 0.3. The left point stays at 1.
4. Press Ctrl+S, then F5: the fire runs white, yellow, orange and grey. Press F5 again to stop.

Colour and size by speed

1. Select Town, press + Add node and choose Particles. Rename it forge_sparks, and set its Position to 114 and 276.
2. Set Lifetime to 1.4, Speed to 110, Direction to 0.3 and -1, Spread to 0.9 and Gravity to 180. Gravity is positive, so the sparks arc up and fall back. Rate is already 20, and End Size already 2, as every new Particles node has them.
3. Set Start Color to 1, 0.9, 0.6, alpha 1, and End Color to 1, 0.4, 0.1, alpha 0.
4. Set Start Size to 4.5, Capacity to 32, Sort Layer to 4 and Blend to Add. Set Texture to soft-dot.png, and Drag to 0.6.
5. Tick Size By Speed and open it. Click the left point and set its y to 0.6, then click the right point and set its y to 1.4.
6. Tick Color By Speed and open it. Add two colour stops: one at 0 set to 1, 0.35, 0.1, a red-orange, and one at 1 set to 1, 1, 0.9, a pale cream. Leave the alpha strip empty: alpha is 1 throughout.
7. Set Speed Range to 0 and 160. Press Ctrl+S.

Size By Speed

A curve that multiplies a particle’s size by how fast it is going at that moment. Its left end is the slowest speed in Speed Range, its right end the fastest. Here 0.6 at the slow end and 1.4 at the fast end: a fast spark is 1.4 times its size and a slow one 0.6 times. 0.5 to 1.5 suits a spark.

Color By Speed

A gradient that tints a particle by its speed, multiplied in as Color Over Life is. Here a red-orange at the slow end and a pale cream at the fast end, so a fast spark runs pale and a slow one reddens. It takes the same colour and alpha stops as Color Over Life.

Speed Range

The two speeds, in world units a second, that the two by-speed shapes read as 0 and 1: the slowest first, the fastest second. One range serves both shapes. Here 0 and 160: a spark leaving at 110 starts about two thirds of the way along, and reddens as Drag and gravity slow it. A particle slower than the first number reads as 0, and one faster than the second as 1. Set the second a little above the fastest speed the emitter makes.

What curves cost

The forge fire and the forge sparks

Check it works