Drawing with shader = only quarter of the screen

[Vovosunt]

I've been using the following sample vertex and frag shaders (comments are not mine):
VERTEX:

varying vec2 vTexCoord;
 
// remember that you should draw a screen aligned quad
void main(void)
{
   //gl_Position = ftransform();
 
   // Clean up inaccuracies
   vec2 Pos;
   Pos = sign(gl_Vertex.xy);
 
   gl_Position = vec4(Pos, 0.0, 1.0);
   // Image-space
   vTexCoord = Pos * 0.5 + 0.5;
}

FRAGMENT:

uniform sampler2D RTBlurH; // this should hold the texture rendered by the horizontal blur pass
varying vec2 vTexCoord;
 
const float blurSize = 1.0/512.0;
 
void main(void)
{
   vec4 sum = vec4(0.0);
 
   // blur in y (vertical)
   // take nine samples, with the distance blurSize between them
   sum += texture2D(RTBlurH, vec2(vTexCoord.x, vTexCoord.y - 4.0*blurSize)) * 0.05;
   sum += texture2D(RTBlurH, vec2(vTexCoord.x, vTexCoord.y - 3.0*blurSize)) * 0.09;
   sum += texture2D(RTBlurH, vec2(vTexCoord.x, vTexCoord.y - 2.0*blurSize)) * 0.12;
   sum += texture2D(RTBlurH, vec2(vTexCoord.x, vTexCoord.y - blurSize)) * 0.15;
   sum += texture2D(RTBlurH, vec2(vTexCoord.x, vTexCoord.y)) * 0.16;
   sum += texture2D(RTBlurH, vec2(vTexCoord.x, vTexCoord.y + blurSize)) * 0.15;
   sum += texture2D(RTBlurH, vec2(vTexCoord.x, vTexCoord.y + 2.0*blurSize)) * 0.12;
   sum += texture2D(RTBlurH, vec2(vTexCoord.x, vTexCoord.y + 3.0*blurSize)) * 0.09;
   sum += texture2D(RTBlurH, vec2(vTexCoord.x, vTexCoord.y + 4.0*blurSize)) * 0.05;
 
   gl_FragColor = sum;
}

The shader works, but when I draw a full-screen sprite with it, only the top-right quarter of the sprite gets drawn:


Any ideas?

[Laurent]

Normalized screen coordinates range from -1 to 1, not from 0 to 1.

[Vovosunt]

Normalized screen coordinates range from -1 to 1, not from 0 to 1.

Oh. Ok, I get it. But I've still got no idea how to fix it.
Making vTexCoord = Pos makes it show continuously shrinking version of the sprite.
Looks cool, but is useless 

[Laurent]

How do you render your fullscreen sprite? If it's a SFML sprite you need to uncomment this line:

//gl_Position = ftransform();

[Vovosunt]

How do you render your fullscreen sprite? If it's a SFML sprite you need to uncomment this line:

//gl_Position = ftransform();

It's an SFML sprite with a texture from a renderTexture. Uncommenting doesn't help

[Laurent]

If you keep the following line, the uncommented one is useless

gl_Position = vec4(Pos, 0.0, 1.0);

What is it supposed to do?

[Vovosunt]

It's just a vertical (non-Gaussian) blur. I've got no idea what am I doing anymore O_O