Graphics Reference
In-Depth Information
seeded values in the
Init
function in the program. The body of the vertex
shader first checks whether a particle's lifetime has expired. If so, the
gl_Position
variable is set to the value (−1000, −1000), which is just a
quick way of forcing the point to be off the screen. Because the point will
be clipped, all of the subsequent processing for the expired point sprites
can be skipped. If the particle is still alive, its position is set to be a linear
interpolated value between the start and end positions. Next, the vertex
shader passes the remaining lifetime of the particle down into the fragment
shader in the varying variable
v_lifetime
. The lifetime will be used in the
fragment shader to fade the particle as it ends its life. The final piece of the
vertex shader causes the point size to be based on the remaining lifetime
of the particle by setting the
gl_Pointsize
built-in variable. This has the
effect of scaling the particles down as they reach the end of their life.
The fragment shader code for the example program is provided in
Example 14-7.
Example 14-7
Particle System Fragment Shader
#version 300 es
precision mediump float;
uniform vec4 u_color;
in float v_lifetime;
layout(location = 0) out vec4 fragColor;
uniform sampler2D s_texture;
void main()
{
vec4 texColor;
texColor = texture( s_texture, gl_PointCoord );
fragColor = vec4( u_color ) * texColor;
fragColor.a *= v_lifetime;
}
The first input to the fragment shader is the
u_color
uniform variable,
which is set at the beginning of each explosion by the
Update
function.
Next, the
v_lifetime
input variable set by the vertex shader is declared
in the fragment shader. In addition, a sampler is declared to which a 2D
texture image of smoke is bound.
The fragment shader itself is relatively simple. The texture fetch uses the
gl_PointCoord
variable as a texture coordinate. This special variable
for point sprites is set to fixed values for the corners of the point sprite
(this process was described in Chapter 7, “Primitive Assembly and
