3D Orthographic Projection - opengl

Question

I want to construct a Orthographic projection to make my sun's shadow map look right. Unfortunately, the code is not achieving the desired results as using the regular perspective projection. Here's my code for setting up the projection matrix:
glViewport (0, 0, (GLsizei)shadowMap.x, (GLsizei)shadowMap.y);
glMatrixMode (GL_PROJECTION);
glLoadIdentity();
//suns use this
glOrtho(0, shadowMap.x, 0, shadowMap.y, 0.1,1000.0);
glMatrixMode (GL_MODELVIEW);
glLoadIdentity();
From what I understand that should be correct. However, after a quick debug render, I noticed that the scene was rendering in a tiny portion of the screen. After some experimentation, I found changing the shadowMap values in glOrtho made it cover the whole texture, but it was really zoomed in. In my perspective projection I use 0.1 and 1000.0 for my near and far, and I've experimented with those and it does change the results, but not get the desired results still. The only time that I get the correct results is when the values are kept with shadowMap.x and shadowMap.y, but like I said, its rendering really small.
What am I doing wrong here? Everything I've read said that the initial code is correct.
EDIT:
Apparently it wasn't clear that this is for the shadow map pass, the regular pass is rendered with perspective and is fine.

Answer 1

Shadow mapping is multi pass algorithm.
You are reffering to the first pass (point 1).
Render scene form light source view into depth texture
Render scene from camera view with depth texture projection mapping enabled.
current fragment xy+depth is then transformed into light projection coordinates and is compared to stored depth on depth texture
if both depths are equal (or nearly equal) current fragment should be considered as lit, otherwise as shadowed.
So everything's fine with your code, store depth values from this pass to depth texture and proceed to point 2.
One thing you should think about is how wide area your light should cover (in world space). With loadidentity on modelview you are attempting to cover 1 world unit x 1 world unit area for you light only.
Consider we have a sphere at 0,0,0 with radius 5.0
We have depth texture of 256,256 dims.
We want to project it along Z onto sphere.
glVieport(0,0,256,256);
glMatrixMode(GL_PROJECTION);
glLoadidentity();
glOrtho(-2.5,2.5,-2.5,2.5,-1000,1000);
glMatrixMode(GL_MODELVIEW);
glLoadidentity();
//flip z, we cast light from obove
glRotate(1,0,0,180);

Answer 2

I don't see where you set the light modelview matrix. You can render a shadow map using the code below:
double* getOrthoMVPmatrix(vector3 position,vector3 lookat,
GLdouble left, GLdouble right,
GLdouble bottom, GLdouble top,
GLdouble nearVal, GLdouble farVal)
{
glPushMatrix();
double projection[16];
double modelView[16];
double *matrix = new double [16];
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho( left, right, bottom, top, nearVal, farVal) ;
glMatrixMode(GL_MODELVIEW);
glEnable(GL_DEPTH_TEST);
glLoadIdentity();
gluLookAt(position.x,position.y,position.z,lookat.x,lookat.y,lookat.z,0,1,0);
glGetDoublev(GL_MODELVIEW_MATRIX, modelView);
glGetDoublev(GL_PROJECTION_MATRIX, projection);
glPopMatrix();
matrix = projection*modelView;
return matrix ;
}
void renderShadowMap(void)
{
//"Bind your depth framebuffer"
glViewport(0,0,"Your SM SIZE","Your SM SIZE");
glEnable(GL_CULL_FACE);
glCullFace(GL_FRONT);
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
double *MVP = getOrthoMVPmatrix( "your light position","your light position" + "your light direction",
-"left","right",
"bottom","top",
"near","far"
) ;
//"call glUseProgram to bind your shader"
// set the uniform MVP we made "
//"Draw your scene "
glViewport(0,0,"screen width","screen height");
}
Your will need to make a multiplication operator for double [16] array. In my case i made a matrix class but do it your way.
Dont forget to call glCullFace(GL_BACK) before drawing your real scene and free MVP after.