Shaders not working as expected - c++

Question

Using OpenGL 3.2 / GLSL 150 with OpenFrameworks v0.8.3
I'm trying to implement shaders into my programs. My program successfully loads the correct frag and vert files, but I get this glitched visual and error:
[ error ] ofShader: setupShaderFromSource(): GL_FRAGMENT_SHADER shader failed to compile
[ error ] ofShader: GL_FRAGMENT_SHADER shader reports:
ERROR: 0:39: Use of undeclared identifier 'gl_FragColor'
I read this SO answer explaining that gl_FragColor is not supported in GLSL 300, but I'm (pretty sure I'm) not using that version. Regardless, when I change gl_FragColor with an outputColor var, my screen just appears black with no error.
Why isn't my shader appearing as expected? I have a feeling it is either my .vert file / a fundamental misunderstanding of how shapes are drawn from within shaders, or versioning problems.
My simplified program:
.h
#pragma once
#include "ofMain.h" //includes all openGL libs/reqs
#include "GL/glew.h"
#include "ofxGLSLSandbox.h" //addon lib for runtime shader editing capability
class ofApp : public ofBaseApp{
public:
void setup();
void draw();
ofxGLSLSandbox *glslSandbox; //an object from the addon lib
};
.cpp
#include "ofApp.h"
//--------------------------------------------------------------
void ofApp::setup(){
// create new ofxGLSLSandbox instance
glslSandbox = new ofxGLSLSandbox();
// setup shader width and height
glslSandbox->setResolution(800, 480);
// load fragment shader file
glslSandbox->loadFile("shader"); //shorthand for loading both .frag and .vert as they are both named "shader.frag" and "shader.vert" and placed in the correct dir
}
//--------------------------------------------------------------
void ofApp::draw(){
glslSandbox->draw();
}
.vert (just meant to be a pass-through... if that makes sense)
#version 150
uniform mat4 modelViewProjectionMatrix;
in vec4 position;
void main(){
gl_Position = modelViewProjectionMatrix * position;
}
.frag (see 3rd interactive code block down this page for intended result)
#version 150
#ifdef GL_ES
precision mediump float;
#endif
out vec4 outputColor;
uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;
float circle(in vec2 _st, in float _radius){
vec2 dist = _st-vec2(0.5);
return 1.-smoothstep(_radius-(_radius*0.01),
_radius+(_radius*0.01),
dot(dist,dist)*4.0);
}
void main(){
vec2 st = gl_FragCoord.xy/u_resolution.xy;
vec3 color = vec3(circle(st,0.9));
outputColor = vec4( color, 1.0 );
}

Answer 1

I don't see the uniform variables getting set anywhere, which means they take the default values. This means that your matrix is all zeroes.