Hi osg-users!
I'm trying to write my first shader, it is a very simple test program which
sets the color on a sphere.
The color depends on whether the sphere has moved relative to its position in
the previous frame.
I use two varying variables, which are set in the vertex shader, see the code
below. If the sphere has moved, the variable called displacementNDC is /= 0.
Then I expect that it should still be /= 0 in the fragment shader, but by some
reason the value is == 0.
The other varying variable, "green", does not change when going from vertex
shader to fragment shader.
(Just for test, I also tried to put the variable called currentPosClip in a
varying variable. Testing for currentPosClip == 0 in the vertex shader returned
false, but it returned true in the fragment shader.)
Please, can anybody explain what is happening? Why are the values changing to
zero?
Regards,
Asa
-----------Vertex shader ------------------
uniform mat4 prevModelViewProjectionMatrix;
varying vec2 displacementNDC;
varying vec4 green;
void main()
{
// Calculate the previous and current position in clip space.
vec4 currentPosClip = gl_ModelViewProjectionMatrix*gl_Vertex; //
Transforms vertex position
vec4 previousPosClip = prevModelViewProjectionMatrix*gl_Vertex; // from
object space to clip space.
gl_Position = ftransform();
// Calculate displacement in NDC space.
vec2 displacementNDC = currentPosClip.xy/currentPosClip.w -
previousPosClip.xy/previousPosClip.w;
// If the displacement is zero, varying variable "green" is set to the RGBA
value for green.
if (displacementNDC == vec2(0,0)) //
Object has not moved.
green = vec4(0, 1, 0, 0);
else
// Object has moved.
green = vec4(0.0);
}
-----------End vertex shader ------------------
-----------Fragment shader ------------------
varying vec2 displacementNDC;
varying vec4 green;
void main()
{
if (displacementNDC == vec2(0,0))
gl_FragColor = vec4(1,0,0,1) + green;
else
gl_FragColor = vec4(0,0,1,1) + green;
}
-----------End fragment shader ------------------
When the sphere has moved, the resulting color should be blue. But it becomes
red.
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