如何在渲染全屏四边形时获取片段着色器中的查看方向

Question

I have to render a scene in 2 steps. First I render into a Frame Buffer Object and I reuse the texture of the FBO for the next step. I render the texture on a fullscreen quad like this with a shader attached:

glMatrixMode(GL_MODELVIEW);
   glPushMatrix();
   glLoadIdentity();
   gluLookAt(0.0,0.0,0.0, 0.0,0.0,-10.0, 0.0,1.0,0.0);

   glMatrixMode(GL_PROJECTION);
   glPushMatrix();
   glLoadIdentity();
   glOrtho(-1.0,1.0,-1.0,1.0,-1.0,1.0);

   glBegin(GL_QUADS);
        glTexCoord2f(0.0,0.0);
        glVertex3f(-1.0,-1.0,0.0);
        glTexCoord2f(1.0,0.0);
        glVertex3f(1.0,-1.0,0.0);
        glTexCoord2f(1.0,1.0);
        glVertex3f(1.0,1.0,0.0);
        glTexCoord2f(0.0,1.0);
        glVertex3f(-1.0,1.0,0.0);
   glEnd();

   glMatrixMode(GL_PROJECTION);
   glPopMatrix();
   glMatrixMode(GL_MODELVIEW);
   glPopMatrix();

In the attached Fragment Shader I need the viewing direction to the actual fragment of the old scene in Eye Space Coords. My first idea was like this:

vec2 uv = gl_TexCoord[0].st;
vec3 viewDirEye = normalize(vec3(uv.x*2.0-1.0, (uv.y)*2.0-1.0, -1.0));

But it seems not working, and I don't know why. It would be great if someone can help me out here.

Answer 1

Your formula does set up a direction vector from the texture coordinates in normalized device coordinates , but not in eye space .

You have to take the projection matrix into account (which will define the horizontal and vertical field of view angle). You can use the inverse projection matrix to transform into eye space.