Get texture color in fragment shader / texture reflected

HI! I have some problem with this basic shader.
I don’t know why, but i try to simply redraw my texture using fragment shader, but if i don’t flip vertically my y coord i see my image reflected.
Can somebody explain me why this happen?
(my image is 1000x400px, like my window)


#include "testApp.h"

void testApp::setup()
	ofBackground(10, 10, 10);
  shader.load( "shader.vert", "shader.frag");

void testApp::update()

void testApp::draw()
  shader.setUniformTexture("ggpTest", ggpTest.getTextureReference(), 1);
  ofRect(0, 0, ofGetWidth(), ofGetHeight());


#version 150

uniform mat4 modelViewProjectionMatrix;

in vec4 position;

void main(){
	gl_Position = modelViewProjectionMatrix * position;


#version 150

out vec4 outputColor;
uniform sampler2DRect ggpTest;

void main()
  vec4 ggpTestColor = texture(ggpTest, vec2(gl_FragCoord.x, 400 - gl_FragCoord.y));
  outputColor = vec4(ggpTestColor);

I have the solution!
If i change the origin for gl_FragCoord​’s window-space, it works without strange tricks!
So, in my fragment shader i added this:

layout(origin_upper_left) in vec4 gl_FragCoord;

Hi @Mauro I’m having this same exact issue. My shader is a bit complex so I hesitate to copy/paste the entire thing here but the important parts are as follows:
.vert file

#version 120

void main()
    gl_Position = ftransform();

.frag file based on shadertoy shader

uniform vec3 iResolution;
uniform float iGlobalTime;
uniform vec4 iMouse;
uniform sampler2D iChannel0; // Texture #1

void main(void)
    vec2 uv =  gl_FragCoord.xy / iResolution.xy;

    vec3 color = vec3(red,green,blue);
    gl_FragColor = vec4(color,1.0);


Where exactly did you place ???

layout(origin_upper_left) in vec4 gl_FragCoord;

Thanks for posting this!