OpenGL GLSL Binding Sampler for Fragment Shader

Question

I am hoping to implement a shader on a 2D OpenGL application. My plan is to render a scene to a framebuffer object, and then render that framebuffer object to the screen using a shader.

Here is the scene, which I have drawn to a framebuffer object, and then to the screen from there. Using the arrow keys makes the moon move around (I am quite proud of it!)

However, when I try to render the framebuffer object to the screen using my shader program, I get this:

It is very sad. This fragment shader is one which I got from a tutorial, and I'm sure the problem has to be with the Uniform Variables.

Here is the Fragment Shader:

#version 330

in vec2 texCoord;
out vec4 outputColor;

uniform sampler2D gSampler;

void main()
{
   outputColor = texture2D(gSampler, texCoord);
}

Here is how I set up the Uniform Variables:

gSampler = glGetUniformLocation(mProgramID, "gSampler");
glUniform1i(gSampler, GL_TEXTURE0);

Here is how I render the Framebuffer (fbo_texture) to the screen, using my shader program (mProgramID)

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT,    0 ); 

glUseProgram(mProgramID);
glColor3f(1.0, 1.0, 1.0);
glEnable(GL_TEXTURE_2D);

glActiveTexture(GL_TEXTURE0 + 0);
glBindTexture(GL_TEXTURE_2D, fbo_texture);

glBegin(GL_QUADS);x1= 0.0;x2= 400.0;y1= 0.0;y2 = 400.0;
    glTexCoord2f(0.0f, 1.0f); glVertex2f(x1, y1);
    glTexCoord2f(0.0f, 0.0f); glVertex2f(x1, y2);
    glTexCoord2f(1.0f, 0.0f); glVertex2f(x2, y2);
    glTexCoord2f(1.0f, 1.0f); glVertex2f(x2, y1);

glEnd();glDisable(GL_TEXTURE_2D);

SDL_GL_SwapBuffers();

So my question is- What is the link between a texture and a sampler? Do I need to render a textured quad, if the shader is already sampling the texture? Why not just render a blank quad if the shader is texturizing it?

EDIT: Here is the vertex shader:

void main()
{

    gl_TexCoord[0] = gl_MultiTexCoord0;
    gl_Position = ftransform();
}

Answer 1

There's indeed a problem with how you set the uniform value for the sampler:

gSampler = glGetUniformLocation(mProgramID, "gSampler");
glUniform1i(gSampler, GL_TEXTURE0);

The value you need to set for the uniform is the index of the texture unit, not the corresponding enum value. Since you are using texture unit 0 in this case, you need to replace the glUniform1i() call by:

glUniform1i(gSampler, 0);