# OpenGL left handed vs right handed coordinate system

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PLEASE look at the my reply in this thread, for the current problem that I am facing ...... DOWN :) I have an application which was coded using a left handed coordinate system. Libraries were self written and all computation was done with left handed system (positive z axis moving into the screen). I want to convert it to openGL coordinate system, which is right handed coordinate system (positive z axis coming out of the screen). The left handed setup of the application:
[Source]
/*
camera position = (0,0, -300);
fov = 0.76, near clipping = 1, far clipping = 1024

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

projection = matrix4x4::perspectiveFovLHGL(fov, (float)w / h, zNear, zFar);	// call to left handed system

matrix4x4 modelview;
modelview =
matrix4x4::scaling(modelScale) *
matrix4x4::rotationQuaternion(modelOrientation) *
matrix4x4::translation(modelPosition) *
matrix4x4::translation(-cameraPosition);

glMatrixMode(GL_MODELVIEW);
*/
[/Source]
[/source] First question: if I change the camera position to (0, 0, 300), I see nothing on the screen. As I am viewing a 3D object, I think that I should be seeing the other face of the object. Right? I tried converting it to the Right handed coordinate system using the following setup:
[Source]
/*
camera position = (0,0, 300);
fov = 0.76, near clipping = 1, far clipping = 1024

glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);
// glFrontFace(GL_CW);
glCullFace(GL_BACK);

projection = matrix4x4::perspectiveFovRHGL(fov, (float)w / h, zNear, zFar);	// call to right handed system like the OpenGL

matrix4x4 modelview;
modelview =
matrix4x4::scaling(modelScale) *
matrix4x4::rotationQuaternion(modelOrientation) *
matrix4x4::translation(modelPosition) *
matrix4x4::translation(-cameraPosition);

glMatrixMode(GL_MODELVIEW);
*/


it does not work :( is there something else that I am missing here to convert best wishes [Edited by - me_here_me on August 6, 2007 11:52:32 AM]

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You need to make sure all your matrices (i.e. your modelview matrix) are RHS rather than LHS. Check out http://www.gamedev.net/community/forums/topic.asp?topic_id=458126

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I did the conversion and the problem was not solved completely. I have tracked down the problem to the following issue:

I have a number of 3d cubes on the screen. These cubes are jumbled up together. I want to generate two textures. First textures records depth map of the front faces of the visible cubes. Second texture records depth map of the back faces of the visible cubes.

This works fine for the left handed system. The setup is shown below:
/*        glEnable(GL_DEPTH_TEST);	glEnable(GL_CULL_FACE);	glFrontFace(GL_CW);	glCullFace(GL_BACK);        projection = matrix4x4::perspectiveFovLHGL(fov, (float)w / h, 		zNear, zFar);        glLoadMatrixf(projection);        glMatrixMode(GL_MODELVIEW);	glTranslatef(0, 0, 379);        glScalef(256, 256, 128);        //----------------------------------	// render depth projection	glViewport(0, 0, fbWidth, fbHeight);	glUseProgram(0);	for (int pass = 0; pass < 2; ++pass) {		switch (pass) {			case 0: // render back depth into framebuffer 0				glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffer0);				glDepthFunc(GL_GREATER);				glClearDepth(0.0f);				glCullFace(GL_FRONT);				break;			case 1: // render front depth into framebuffer 1				glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffer1);				glDepthFunc(GL_LESS);				glClearDepth(1.0f);				glCullFace(GL_BACK);				break;		}		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 tried but i am not able to convert it to right handed system. I did the following but it just not goes fine. The following is the setup. I have marked with ** the lines which are different from code snippet above:
/*        glEnable(GL_DEPTH_TEST);	glEnable(GL_CULL_FACE);**   //	glFrontFace(GL_CW);	glCullFace(GL_BACK);**      projection = matrix4x4::perspectiveFovRHGL(fov, (float)w / h, 		zNear, zFar);        glLoadMatrixf(projection);        glMatrixMode(GL_MODELVIEW);**	glTranslatef(0, 0, -379);        glScalef(256, 256, 128);        //----------------------------------	// render depth projection	glViewport(0, 0, fbWidth, fbHeight);	glUseProgram(0);	for (int pass = 0; pass < 2; ++pass) {		switch (pass) {			case 0: // render back depth into framebuffer 0				glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffer0);				glDepthFunc(GL_GREATER);				glClearDepth(0.0f);				glCullFace(GL_FRONT);				break;			case 1: // render front depth into framebuffer 1				glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebuffer1);				glDepthFunc(GL_LESS);				glClearDepth(1.0f);				glCullFace(GL_BACK);				break;		}		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 am calculating cubes using a library at another stage

I need help converting this piece of code

thanks in abvance

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Quote:
 I did the following but it just not goes fine.

What exactly doesn't work?

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framebuffer0 with back depth is filled with all 0

framebuffer1 with front depth is filled with all 1

so it does not change any value from where it all started.

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So nothing is being drawn then.... I suspect your camera is set up incorrectly.

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

is probably the problem. What is in this function?

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sorry for a late reply. I was out of place

here is the function

inline matrix4x4 matrix4x4::perspectiveFovRHGL( float fovY, float aspect, float zn, float zf ) {	float h = 1.0f / tan(fovY / 2.0f);	float w = h / aspect;	return matrix4x4(w, 0, 0,                0,			 0, h, 0,                0,			 0, 0, (zn+zf)/(zn-zf), -1,			 0, 0, 2*zn*zf/(zf-zn),  0);}

matrix is column ordered.

best wishes

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u seem right, actually i have trouble even converting it to simple parallel projection. there is some serious problem in the setup ... :(

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I suggest setting up a very simple camera using OpenGL functions and rendering to screen and then fiddle around with that until everything is working as it should. Good luck!

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