CODEWITHSUNDEEP
javaopenglmathopengl-es
LessComplexity
LessComplexity

Reputation: 490

Matrix-vector Multiplication

say for example I have a class named Matrix4x4 and the class has the following function:

public float[] MultiplyVec(float[] vec){
    float[] newV = new float[4];

    for(int i = 0; i < 4; i++){
        int value = 0;
        for(int j = 0; j < 4; j++){
            value += matrix[i][j] * vec[j]; 
        }
        newV[i] = value;
    }

    return newV;
}

And in another class I have an object name modelMatrix that is an instance of Matrix4x4 which is initialized this way:

public Matrix4x4 modelMatrix = Matrix4x4.IdentityM();

Where the IdentityM() function contains:

public static Matrix4x4 IdentityM(){
        return new Matrix4x4(new float[][]{
                {1f, 0f, 0f, 0f},
                {0f, 1f, 0f, 0f},
                {0f, 0f, 1f, 0f},
                {0f, 0f, 0f, 1f}
        });
}

Now I have a little problem that I can't figure out. let's say for example that I have a rectangle with x, y, width and height. and now for example I do:

modelMatrix.translate(0.002f, 0.002f, 0);

Where the function translate contains:

public void translate(float x, float y, float z){
        matrix[0][3] += x;
        matrix[1][3] += y;
        matrix[2][3] += z;
}

Using OpenGL ES when I send the modelMatrix to the shader, the object moves correctly, but when I calculate the vertices with modelMatrix, they stay the same and don't change, the function that calculates the vertices is:

public float[] getVertices(){
        // Vertices in object space:
        float[] vector1 = {x, y, 0f, 1f};
        float[] vector2 = {x + width, y, 0f, 1f};
        float[] vector3 = {x + width, y + height, 0f, 1f};
        float[] vector4 = {x, y, 0f, 1f};

        // Calculate the vertices in world space:
        float[] vector1r = modelMatrix.MultiplyVec(vector1);
        float[] vector2r = modelMatrix.MultiplyVec(vector2);
        float[] vector3r = modelMatrix.MultiplyVec(vector3);
        float[] vector4r = modelMatrix.MultiplyVec(vector4);

        if(LoggerConfig.ON) Log.d("Middle Vertex", vector1r[2] + ", "
                                                 + vector1r[3]);

        return new float[] {
                vector1r[0], vector1r[1], 
                vector2r[0], vector2r[1], 
                vector3r[0], vector3r[1], 
                vector4r[0], vector4r[1]
        };
    }

And the results in the log stay the same even though I move the object and the object moves in OpenGL, is this a problem with the calculation I'm doing? How can I correct it?

Upvotes: 2

Views: 2090

Answers (1)

Reto Koradi
Reto Koradi

Reputation: 54592

In your MultiplyVec() method, you're rounding all the intermediate values to integers:

for(int i = 0; i < 4; i++){
    int value = 0;
    for(int j = 0; j < 4; j++){
        value += matrix[i][j] * vec[j]; 
    }
    newV[i] = value;
}

Since value is a variable of type int, all the partial sums will be rounded to integers. You need to use a float variable for the intermediate value:

for(int i = 0; i < 4; i++){
    float value = 0f;
    ...

Upvotes: 3

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