vec2 Coord = v_vTexcoord;
gl_FragColor = v_vColour * texture2D( gm_BaseTexture, Coord);
This is a pass-through shader. If you run the shader applied to a sprite, it shouldn't change anything. To make a ripple we will use a cosine wave. We could do it like this:
vec2 Coord = v_vTexcoord + vec2(cos(v_vTexcoord.y*30.0)/30.0,0);
If you run this you will see this (before and after):
vec2 Coord = v_vTexcoord + vec2(cos(v_vTexcoord.y*30.0+Time*6.2831)/30.0,0);
It is multiplied by 6.2831 (pi times two) because cosine uses angles in radians so that it does a full cycle every second.
We'll set the time uniform in GameMaker like this in the draw event:
var uTime = shader_get_uniform(shdr_wind,"Time");
This will set "Time" to the number of seconds since the game started.
If you run this it will be animated. You may notice a problem where the base of the grass moves
just as much as the top.
Fixing and perfecting
vec2 Coord = v_vTexcoord + vec2(cos(v_vTexcoord.y*30.0 + Time*6.2831)/30.0,0)*(1.0-v_vTexcoord.y);
It looks much better now. This shader is currently proportion to the sprite size. If it's long it will stretch the waves, but if it's short it will shrink the waves. That is because the waves are based off of the v_vTexcoord which is between 0 and 1. To fix this we can use the position attribute from the vertex shader because it uses the vertex coordinates rather than the texture coordinates.
We need to add a varying vec2 called "v_vPosition" in the vertex and fragment shaders by the other varying vectors. Set it in the vertex shader like this:
v_vPosition = in_Position.xy;
And back to the fragment shader use this for the coordinates:
vec2 Size = vec2(256,128);
vec2 Wave = vec2(48,5);
vec2 Coord = v_vTexcoord + vec2(cos((v_vPosition.y/Wave.x+Time)*6.2831)*Wave.y,0)/Size*(1.0-v_vTexcoord.y);
"Size" should be the size of the texture. Use a uniform if necessary. "Wave" is for adjusting the wave shape. The first number is the height of the wave and the second number is the amplitude.
This works by making a cosine wave of the chosen size with time added (multiplied by 6.2831 because of radians) and then multiplied by the amplitude; then divided by "Size" to get it between 0 and 1 and finally multiplied by the code I mentioned above to fix the bottom.
Thanks for reading.
Basic tutorials used:
#4 Vertex Shader
#2 Black and White Shader
#1 Shader Basics