View Full Version : volume rendering light problem

03-04-2006, 03:55 AM
Some days ago, I tried to achieve a headlight effect for my volume rendering.But the light's position is always wrong after rotating the object. Thanks for guys in this forum, but the problem still exists. :(

I send the fixed light position into the shader then calculate phong light in the shader.But the light position seems to change when rotating the object. So I try to transform the normal to eye space or transform the light position into model space. BUT all of them doesn't work in my project!

I have some comments , please tell me what's wrong in it.

Method 1. I use vec3 NN = normalize(gl_NormalMatrix * myNormalTexure.xyz); to transform normals.
These codes does work when rotating along x and y axis, but it goes wrong when rotating
along z-axis.
Method 2. I use vec3 lightVec = vec3(-gl_NormalMatrix[0][2],-gl_NormalMatrix[1][2],-gl_NormalMatrix[2][2]);
to transform light position.But the light stay fixed relative to my scene. I don't know why it doesn't work.

Any suggestion will be warmly welcomed.
thanks in advance :)

03-06-2006, 08:50 AM
Are u using the glLightfv command to set the light pos? if u are specifying the light position after u set the rotation matrix the state.light[n].position will contain the transformed light position ,..... am not sure what method u are using

03-06-2006, 03:04 PM
thank you for your reply
I'm not using glLightfv() actually. Here is my codes snippet outside the shader.

When initialize the window ,I use OpenGL tranformation as this:
glTranslatef(0 , 0 , -2);

Then in my Redraw() function, my code goes like this:
shader->sendUniform3f(...); //send light pos
/*-------m_AR is an object for my arcball class which is used to calculate the translation and rotation and scale ------------*/

03-06-2006, 07:29 PM
I had used arcball method some time ago ,, it gave me problems with per vertex lighting .. the scene used to go black at certain angles ,, the model view matrix was getting some scale factor .. but guess my arcball code was wrong ,,i dumped it for a diff method. . y dont u try the following just to check if the Z-rotation u said is causing the problem ..

comment out the arc_ball transformation.
Call glRotate to rotate it about the z-axis .. and see whether the light still moves with the scene ..
also try using simple per vertex lighting to just check out if the arc works fine ,,,