Simple texture advection: texture flow shader

flowShader.frag

#version 150

// CONSTANTS
#define DEBUG_MODE

// UNIFORMS
uniform sampler2D tex0;	// flow field texture
uniform sampler2D tex1; // texture A for swapping
uniform sampler2D tex2; // texture B for swapping
uniform float time;
uniform float cycle;

// VARIYNG FROM VERTEX SHADER
in vec2 vTexCoord; // uv coords

// FRAGMENT SHADER OUTPUT
out vec4 outputColor;

vec2 unpackFlowVector(vec2 flowVecData)
{
    vec2 flowVector = flowVecData * 2.0 - 1.0;
    return flowVector;
}
 
void main()
{
    float halfCycle = cycle * 0.5;
    float flowMapOffset0 = mod(time/20, cycle);
    float flowMapOffset1 = mod((time/20) + halfCycle, cycle)
    
    vec4 color = vec4( 1.0, 0.0, 0.0, 1.0 );

    vec4 flowMapTexel = texture(tex0, vTexCoord);
    vec2 flowDir = unpackFlowVector(flowMapTexel.rg);
	
    vec4 texelA = texture(tex1, vTexCoord + flowDir * flowMapOffset0); // texture2D is deprecated since version 1.30
    vec4 texelB = texture(tex2, vTexCoord + flowDir * flowMapOffset1);

    float mixFactor = (abs(halfCycle - flowMapOffset0) / halfCycle);
    color = mix(texelA, texelB, mixFactor);
    
#ifdef DEBUG_MODE
    outputColor = mix(color, flowMapTexel, 0.3);
#else
    outputColor = color;
#endif
}

flowShader2.frag

/*
In this version we expect to calculate the texture offset from cpu
*/
#version 150

// CONSTANTS
#define DEBUG_MODE

// UNIFORMS
uniform sampler2D tex0;	// flow field texture
uniform sampler2D tex1; // texture A for swapping
uniform sampler2D tex2; // texture b for swapping
uniform float time;
float halfCycle;

//Flow map offsets used to scroll the wave maps
uniform float flowMapOffset0;
uniform float flowMapOffset1;

// VARIYNG FROM VERTEX SHADER
in vec2 vTexCoord; // uv coords

// FRAGMENT SHADER OUTPUT
out vec4 outputColor;

vec2 unpackFlowVector(vec2 flowVecData)
{
    vec2 flowVector = flowVecData * 2.0 - 1.0;
    return flowVector;
}
 
void main()
{
    vec4 color = vec4( 1.0, 0.0, 0.0, 1.0 );

    vec4 flowMapTexel = texture(tex0, vTexCoord);
    vec2 flowDir = unpackFlowVector(flowMapTexel.rg);
	
    vec4 texelA = texture(tex1, vTexCoord + flowDir * flowMapOffset0); // texture2D is deprecated since version 1.30
    vec4 texelB = texture(tex2, vTexCoord + flowDir * flowMapOffset1);

    float mixFactor = (abs(halfCycle - flowMapOffset0) / halfCycle);
    color = mix(texelA, texelB, mixFactor);
    
#ifdef DEBUG_MODE
    outputColor = mix(color, flowMapTexel, 0.3);
#else
    outputColor = color;
#endif
}