Setting light position (trap report)

OpenGL Development JOGL Development
#1

Hi there,

This is not a question but a warning of a trap, and probably a request for a jogl feature. Those who can cite the OpenGL specification from memory probably won’t find this very entertaining, but still, I found it worth mentioning after investing two hours into debugging :slight_smile:

I recently enabled lighting operations in my jogl-based engine and observed some weird behavior, with lit faces flickering between almost correct result color and ambient base color randomly.

I had initialized my light’s position like this …

a_gl.glLightfv(GL.GL_LIGHT0, GL.GL_POSITION, new float[]{10f,10f,10f});

… assuming that a 3-vector would be required. Of course, this is wrong, light position is set in homogenous coordinates, the correct call being …

a_gl.glLightfv(GL.GL_LIGHT0, GL.GL_POSITION, new float[]{10f,10f,10f,1f});

Now the problem is that jogl did tell me exactly nothing about this. I mean, one would exepect some range checking and an error message, or at least a crash, but jogl silently accepts the too-short array and obviously sets the missing parameter … to 0, which is fatal if you are working with homogenous coordinates, if a default value then it should be 1, which would allow people to pass 3-Vectors and get valid results.

So newbies (like me :wink: ) don’t fall for it, and jogl-team, if you find the time, think of something.

Cheers

Wolfgang

#2

Why should JOGL be doing the checking? This is standard behaviour of OpenGL. If you’re going to start putting checks in there, then you may as well build a high level API like Java3D.

#3

Just so as you know, setting the 4th coordinate to 0 makes the light purely directional, which makes lighting calculations a bit faster.

#4

Okay, I had that coming for not being more specific ;D

I know about the fourth coordinate and its meaning. I was talking about it being omitted, passing a too-short array. What happens in C when you issue glLight with GL_POSITION and a 3-float array? I assume some kind of error will be thrown? If not, then of course forget about my suggestion, JOGL shouldn’t do more than emulate standard OpenGL behavior, I didn’t ask for more.

Cheers

Wolfgang

#5

Since C has no array-boundary functionality at all, it just takes whatever there is next in memory as 4th component…