7 replies to this topic

[me_here_me]

Can some one help me convert the following code from the left handed coordinate system to right handed coordinate system (the one that OpenGL uses).

I am kindda stuck into it for quite some time now:

below is the working code in the left handed coordinate system:

        glEnable(GL_DEPTH_TEST);
	glEnable(GL_CULL_FACE);
	glFrontFace(GL_CW);

        projection = matrix4x4:: perspectiveFovLHGL(fov, (float)w / h, 
		zNear, zFar);
        glLoadMatrixf(projection);

        glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	
        glTranslatef(0,0, 300);    // should be (0,0,-300) in RightHanded system
	glScalef(256, 256, 128);

        glViewport(0, 0, fbWidth, fbHeight);
	glUseProgram(0);

	
	// render back depth into framebuffer 0
	glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffer0);
	glDepthFunc(GL_GREATER);
	glClearDepth(0.0f);
	glCullFace(GL_BACK);
	
	glClear(GL_DEPTH_BUFFER_BIT);

	if (volume.cubeQuads > 0) {
		glBindBuffer(GL_ARRAY_BUFFER, volume.vbCubes);
		glVertexPointer(3, GL_FLOAT, 0, 0);
		glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, volume.ibCubes);
		glEnableClientState(GL_VERTEX_ARRAY);

		glDrawElements(GL_QUADS, volume.cubeQuads * 4, GL_UNSIGNED_INT, 0);

		glDisableClientState(GL_VERTEX_ARRAY);
		glBindBuffer(GL_ARRAY_BUFFER, 0);
		glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
	}

I have to use glTranslate(0,0,-300) rather then glTranslate(0,0,300) in line 9.

I tried doing it by changing the following lines:
glFrontFace(GL_CW); TO glFrontFace(GL_CCW);

projection = matrix4x4:: perspectiveFovLHGL(fov, (float)w / h,
zNear, zFar);
TO
projection = matrix4x4:: perspectiveFovRHGL(fov, (float)w / h,
zNear, zFar); (is a correct library call)

glTranslatef(0,0, 300); TO glTranslatef(0,0, -300);


AND

case 0: // render back depth into framebuffer 0
	glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffer0);
	glDepthFunc(GL_GREATER);
	glClearDepth(0.0f);
	glCullFace(GL_BACK);
	break;

TO

case 0: // render back depth into framebuffer 0
	glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffer0);
	glDepthFunc(GL_LESS);
	glClearDepth(0.0f);
	glCullFace(GL_FRONT);
	break;

it does not work :(

I posted this on another forum but could not get any help :(

[.oisyn]

What are your znear and zfar values?

[me_here_me]

znear and zfar are 1 and 1024 respectively.

[.oisyn]

It's been a long time since I've done anything with opengl, but shouldn't those values be -1 and -1024 for a righthanded system?

[me_here_me]

actually znear and zfar have to be always positive in openGL.

they are used in the projection matrix later on and thus setup correctly by the openGL itself.

It seems to me that the problem exists somewhere in the following lines

	glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffer0);

	glDepthFunc(GL_LESS);

	glClearDepth(0.0f);

	glCullFace(GL_FRONT);

I cannot figure out how can I set them up. glClearDepth only gets valuess betwee [0, 1].

thanks for the reply

[Kenneth Gorking]

With that setup, you are telling the GPU to only accept pixels with a depth value less than zero, ie. behind the near plane :)

Using glClearDepth(1.0f) and glDepthFunc(GL_LEQUAL) should do the trick.

[me_here_me]

it does not work either :(

in the RHS my objects are on the negative side of the z-axis. So this means that negative values are expected to be stored. openGL system seems to look between znear and zfar, which are both positive.

i think I have to live with the LHS :(

thanks for the help

[Reedbeta]

In OpenGL it's expected that visible objects will have negative Z in eye space, since the Z axis points out of the screen. (The znear and zfar values should be positive nonetheless.)

In any case, I believe you'll need to scale everything by (1, 1, -1), won't you?

Also, don't change CullFace to front, keep it as back. The culling mode is already switched when you change FrontFace from CW to CCW.