Perspective projection matrix in OpenGL

Question

I an new to perspective division phenomenon. I am rendering a simple square in 3D space using following code :

    void MatrixPersp(Mat4& matrix,const float fovy, const float aspect, const float zNear, const float zFar)
    {
         float sine, cotangent, deltaZ;
         float radians;

         for(int i=0;i<4;i++)
             for(int j=0;j<4;j++)
             {
                 if(i==j)
                    matrix[i][j]=1.0;
                 else
                     matrix[i][j]=0.0;
             }

         radians = fovy / 2 * GLES_Pi / 180;

         deltaZ = zFar - zNear;
         sine = (float) sin(radians);

         cotangent = (float) cos(radians) / sine;

         matrix[0][0] = cotangent / aspect;
         matrix[1][1] = cotangent;
         matrix[2][2] = -(zFar + zNear) / deltaZ;
         matrix[2][3] = -1;
         matrix[3][2] = -2 * zNear * zFar / deltaZ;
         matrix[3][3] = 0;

         return;    
    }

void Render()
{
    GLfloat vertices[] = 
    {
        -0.8,0.6,1.0,1.0,
        -0.8,0.2,1.0,1.0,
         0.2,0.2,1.0,1.0,
        0.2,0.6,1.0,1.0
    };

    MatrixPersp(perspective,90.0,aspect,2.0,100.0);

    glUniformMatrix4fv(glGetUniformLocation(program_object,"MVPMatrix"), 1, GL_FALSE, (GLfloat*)perspective);

    glClearDepth(1.0f);
    glClear(GL_DEPTH_BUFFER_BIT);

    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LEQUAL);


glDrawElements();

}

Vertex Shader is :

#version 150

in vec4 position;

uniform mat4 MVPMatrix;

void main()
{
        gl_Position = position*MVPMatrix;

}

The problem here is that , when all the four vertices have same z-value nothing is rendered at all. On the other hand if two vertices have -1 as z-coordinate the projection matrix works fine. I am not sure what is going wrong here.

Answer 1

glUniformMatrix4fv(glGetUniformLocation(program_object,"MVPMatrix"), 1, GL_FALSE, (GLfloat*)perspective);

This line suggests that your matrix is in column-major order. This is confirmed by the way your MatrixPersp function computes its matrix, depending on exactly how Mat4 is defined.

gl_Position = position*MVPMatrix;

If MVPMatrix is properly column-major, then this multiplication is backwards. The position goes on the right.