glsl bind computed fragment shader to cube face - opengl-es

Question

I have a cube , i want to bind a computed fragment shader to a cube face. any help?
my vertex shader is :
precision highp float;
attribute vec3 position;
attribute vec3 normal;
uniform mat3 normalMatrix;
uniform mat4 modelViewMatrix;
uniform mat4 projectionMatrix;
varying vec3 fNormal;
varying vec3 fPosition;
void main()
{
fNormal = normalize(normalMatrix * normal);
vec4 pos = modelViewMatrix * vec4(position, 1.0);
fPosition = pos.xyz;
gl_Position = projectionMatrix * pos;
}
and my fragment shader:
precision highp float;
uniform float time;
uniform vec2 resolution;
varying vec3 fPosition;
varying vec3 fNormal;
void main()
{
float minDimension = min(resolution.x, resolution.y);
vec2 bounds = vec2(resolution.x / minDimension, resolution.y / minDimension);
vec2 uv = gl_FragCoord.xy / minDimension;
vec2 midUV = vec2(bounds.x * 0.5, bounds.y * 0.5);
uv.xy-=midUV;
float dist = sqrt(dot(uv, uv));
float t = smoothstep(0.2+0.1*cos(time), 0.1-0.01, dist);
vec4 disc_color=vec4(1.0,0.,0.,1.);
vec4 bkg_color=vec4(0.0, 0.0, 0.0, 1.0);
vec4 resultColor;
resultColor = mix(bkg_color, disc_color, t);
gl_FragColor = resultColor;
}
i get a cube with a dot on the hole image.
but what i want is actually to have the dot on each face.
you can test the code directly under http://shdr.bkcore.com/

Answer 1

The standard approach to do this in OpenGL is to first render your content to a FBO (Frame Buffer Object). This allows you to render to a texture. You can then use this texture for one of the sides when rendering the cube.
The following is not complete code, but just an outline to give you a starting point:
// create a texture, and bind it
GLuint texId = 0;
glGenTextures(1, &texId);
glBindTexture(GL_TEXTURE_2D, texId);
// set texture parameters (filters, wrap mode, etc)
glTexParameteri(GL_TEXTURE_2D, ...);
// allocate texture data (no data provided)
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_FLOAT, 0);
glBindTexture(GL_TEXTURE_2D, texId);
// create and set up FBO with texture as attachment
GLuint fboId = 0;
glGenFramebuffers(1, &fboId);
glBindFramebuffer(GL_FRAMEBUFFER, fboId);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texId, 0);
// create a depth renderbuffer and attach it if depth testing is needed
// start rendering
glViewport(0, 0, width, height);
glClear(GL_COLOR_BUFFER_BIT);
glDraw...(...);
// back to default framebuffer
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glViewport(0, 0, windowWidth, windowHeight);
glClear(GL_COLOR_BUFFER_BIT);
// render cube using texture we rendered to
glBindTexture(GL_TEXTURE_2D, texId);
// rest of state setup and rendering

Answer 2

thx for the answer, this is right, but i found a better way then rendering to a texture. i passed the texture coordinates and used them instead of the glfragcoord in the fragment shader.