CODEWITHSUNDEEP
c++openglglslglm-math
user974125
user974125

Reputation: 21

Math performed on CPU has different result on GPU

I am attempting to have a GLSL fragment shader distort incoming fragments based on their texture coordinates to poorly simulate a CRT.

After the code failed to work, I ported it to C++ to modify the RGB values of a texture. The code worked as expected.

This brings me to believe that something is wrong with my GLSL code, even though it is mirrored in C++ and works perfectly.

Is there something that I don't know about GLSL math that could be causing this?

C++ code

const unsigned int RED = 0xFFFF0000;
const unsigned int BLUE = 0xFF0000FF;
const float X_MAX = 429.0f/448.0f;
const float Y_MAX = 320.0f/336.0f;
const float X_CORNER = 410.0f/448.0f;
const float Y_CORNER = 306.0f/336.0f;
const float X_COEF = (X_MAX-X_CORNER) / (Y_CORNER * Y_CORNER);
const float Y_COEF = (Y_MAX-Y_CORNER) / (X_CORNER * X_CORNER);
float FUNCX(float y)
{
    return X_MAX-X_COEF*y*y;
}
float FUNCY(float x)
{
    return Y_MAX-Y_COEF*x*x;
}

unsigned int get(glm::vec2 intex)
{
    intex *= 2.0;       // Transform the texture rectangle from 0..1
    intex.x -= 1.0;     //                  to
    intex.y -= 1.0;     //              -1 .. 1
    glm::vec2 d = glm::vec2(0.0,0.0);
    d.x = FUNCX(intex.y); // get the curve amount for X values based on Y input
    d.y = FUNCY(intex.x); // get the curve amount for Y values based on X input
    if (abs(intex.x/d.x) > 1.0) // if the X value is outside of the curve
        return RED;             // draw RED for debugging
    if (abs(intex.y/d.y) > 1.0) // if the Y value is outside of the curve
        return BLUE;            // draw BLUE for debugging
    glm::vec2 outtex = glm::vec2(0.0f,0.0f);
    outtex.x = 1.0 + intex.x/d.x; // Now the -1 .. 1 values get shifted back
    outtex.y = 1.0 + intex.y/d.y; //                to
    outtex /= 2.0;                //               0 .. 1
    return texture.get(512*outtex.x,512*outtex.y);
}

GLSL fragment shader:

const vec4 RED = vec4(1.0,0.0,0.0,1.0);
const vec4 BLUE = vec4(0.0,0.0,1.0,1.0);
const float X_MAX = 429.0/448.0;
const float Y_MAX = 320.0/336.0;
const float X_CORNER = 410.0/448.0;
const float Y_CORNER = 306.0/336.0;
const float X_COEF = (X_MAX-X_CORNER) / (Y_CORNER * Y_CORNER);
const float Y_COEF = (Y_MAX-Y_CORNER) / (X_CORNER * X_CORNER);
float FUNCX(float y)
{
    return X_MAX-X_COEF*y*y;
}
float FUNCY(float x)
{
    return Y_MAX-Y_COEF*x*x;
}

vec4 get(vec2 intex)
{
    intex *= 2.0;       // Transform the texture rectangle from 0..1
    intex.x -= 1.0;     //                  to
    intex.y -= 1.0;     //              -1 .. 1
    vec2 d = vec2(0.0,0.0);
    d.x = FUNCX(intex.y); // get the curve amount for X values based on Y input
    d.y = FUNCY(intex.x); // get the curve amount for Y values based on X input
    if (abs(intex.x/d.x) > 1.0) // if the X value is outside of the curve
        return RED;             // draw RED for debugging
    if (abs(intex.y/d.y) > 1.0) // if the Y value is outside of the curve
        return BLUE;            // draw BLUE for debugging
    vec2 outtex = vec2(0.0,0.0);
    outtex.x = 1.0 + intex.x/d.x; // Now the -1 .. 1 values get shifted back
    outtex.y = 1.0 + intex.y/d.y; //                to
    outtex /= 2.0;                //               0 .. 1
    return texture2D(texture,outtex);
}

Note: The Super Mario World image is for testing purposes only. Note 2: The 512 values in the C++ code are the size of the texture used.

Edit: There was a typo in the GLSL code where a y value was divided by an x instead of y. This has been fixed and the new output is the same because the denominator values are so close.

Upvotes: 0

Views: 188

Answers (1)

Reto Koradi
Reto Koradi

Reputation: 54642

The code is not the same. In the C++ code:

if (abs(intex.y/d.y) > 1.0) // if the Y value is outside of the curve
    return BLUE;            // draw BLUE for debugging

In the GLSL code:

if (abs(intex.y/d.x) > 1.0) // if the Y value is outside of the curve
    return BLUE;            // draw BLUE for debugging

The C++ version divides by d.y, the GLSL version by d.x.

Upvotes: 3

Related Questions