How to convert world coordinates to screen coordinates in OpenGL ES 2.0

Question

I am using following OpenGL ES 1.x code to set my projection coordinates.

glMatrixMode(GL_PROJECTION); 

float width = 320;
float height = 480;

glOrthof(0.0,                  // Left
         1.0,                  // Right
         height / width,       // Bottom
         0.0,                  // Top
         -1.0,                 // Near
         1.0);                 // Far
glMatrixMode(GL_MODELVIEW);

What is the equivalent method to setup this in OpenGL ES 2.0 ? What projection matrix should I pass to the vertex shader ?

I have tried following function to create the matrix but its not working:

void SetOrtho (Matrix4x4& m, float left, float right, float bottom, float top, float near, 
float far)
{
    const float tx = - (right + left)/(right - left);
    const float ty = - (top + bottom)/(top - bottom);
    const float tz = - (far + near)/(far - near);

    m.m[0] = 2.0f/(right-left);
    m.m[1] = 0;
    m.m[2] = 0;
    m.m[3] = tx;

    m.m[4] = 0;
    m.m[5] = 2.0f/(top-bottom);
    m.m[6] = 0;
    m.m[7] = ty;

    m.m[8] = 0;
    m.m[9] = 0;
    m.m[10] = -2.0/(far-near);
    m.m[11] = tz;

    m.m[12] = 0;
    m.m[13] = 0;
    m.m[14] = 0;
    m.m[15] = 1;
}

Vertex Shader :

uniform mat4 u_mvpMatrix;

attribute vec4 a_position;
attribute vec4 a_color;

varying vec4 v_color;

void main()
{
   gl_Position = u_mvpMatrix * a_position;
   v_color = a_color;
}

Client Code (parameters to the vertex shader):

float min = 0.0f;
float max = 1.0f;
const GLfloat squareVertices[] = {
    min, min,
    min, max,
    max, min,
    max, max
};
const GLfloat squareColors[] = {
    1, 1, 0, 1,
    0, 1, 1, 1,
    0, 0, 0, 1,
    1, 0, 1, 1,
};
Matrix4x4 proj;
SetOrtho(proj, 0.0f, 1.0f, 480.0/320.0, 0.0f, -1.0f, 1.0f );

The output i am getting in the iPhone simulator:

enter image description here

You have updated your question, but have actually seen and understood Tommy's answer? — Christian Rau
@ChristianRau Yes, I've tried the transpose flag but it doesn't work. I am not applying any other matrix except the one mentioned above. I want to get the Ortho projection right, but it seems like I am getting perspective projection! — kal21

Tommy Tommy · Accepted Answer · 2011-10-17T12:18:28

Your transcription of the glOrtho formula looks correct.

Your Matrix4x4 class is custom, but is it possible that m.m ends up being loaded directly as a glUniformMatrix4fv? If so check that you're setting the transpose flag as GL_TRUE, since you're loading data in row major format and OpenGL expects column major (ie, standard rules are that index [0] is the top of the first column, [3] is at the bottom of the first column, [4] is at the top of the second column, etc).

It's possibly also worth checking that —— assuming you've directly replicated the old world matrix stacks — you're applying modelview and projection in the correct order in your vertex shader or else compositing them correctly on the CPU, whichever way around you're doing it.

How to convert world coordinates to screen coordinates in OpenGL ES 2.0

1 Answers