translate-scroll repeated 2d texture using opengl shader

Question

I managed to implement an infinite 2d texture scrolling using the following shader. btw I'm using cocos2d-x 2.2.1.

The vertex shader:

attribute vec4 a_position;
attribute vec2 a_texCoord;

#ifdef GL_ES
varying mediump vec2 v_texCoord;
#else
varying vec2 v_texCoord;
#endif

void main()
{
    gl_Position = CC_MVPMatrix * a_position;
    v_texCoord = a_texCoord;
}

The fragment shader:

#ifdef GL_ES
precision lowp float;
#endif

uniform vec2 u_time;
uniform vec4 u_color;
varying vec2 v_texCoord;
uniform sampler2D CC_Texture0;

void main()
{
        gl_FragColor = texture2D(CC_Texture0, v_texCoord + u_time);
}

But once I try to add some uniform color the texture is no more repeated correctly (and performance decreases drastically):

/// Texture not repeated correctly and performance degradation 
gl_FragColor = texture2D(CC_Texture0, v_texCoord + u_time) * u_color; 

However the following repeat the texture correctly (but of course there's no scrolling)

gl_FragColor =  texture2D(CC_Texture0, v_texCoord) * u_color; 

What am I doing wrong?

Answer 1

I solved the problem by updating the texture coordinate in the vertex shader.

attribute vec4 a_position;
attribute vec2 a_texCoord;

uniform vec2 u_time;

#ifdef GL_ES
varying mediump vec2 v_texCoord;
#else
varying vec2 v_texCoord;
#endif

void main()
{
    gl_Position = CC_MVPMatrix * a_position;
    v_texCoord = a_texCoord + u_time;
}

Although it ran correctly on Android, updating the texture coordinate in the fragment shader didn't work on iOS.

Answer 2

The problem is probably:

precision lowp float;

Which causes u_time to be 8bit unsigned fixed points with a range of only 0 to 1 or -2 to 2.

setting it to mediump would probably fix the scrolling issue but would still degrade performance greatly.

uniform mediump vec2 u_time;
uniform lowp vec4 u_color;

It's better to calculate UV coordinates in the vertex shader anyway as calculating them in the pixel shader prevents texture prefetching at the interpolation stage and stalls the GPU while it waits for the texture data in the pixel shader.