Sean's blog

Trying out JBullet: IntBuffer is not direct

JBullet Basic Demo

I’ve been simulating the movement of a line-following robot in plain old Java recently, but wanted to try a few problems that need physics, so I downloaded JBullet – a Java port of the Bullet  Physics Library. It needs LWJGL – the Lightweight Java Game Library – to run its demos, so I installed that in exactly the way they say you don’t need to and ran into this Exception when I tried running the JBullet demos/basic/BasicDemo.java (I setup my PC so that I can right-click on Java files in ROX-Filer and choose ‘File..theJVM’) – here’s the stack trace:

Exception in thread "main" java.lang.IllegalArgumentException: IntBuffer is not direct
at org.lwjgl.BufferChecks.checkDirect(BufferChecks.java:127)
at org.lwjgl.opengl.GL11.glGenTextures(GL11.java:1459)
at com.bulletphysics.demos.opengl.FontRender$GLFont.load(FontRender.java:124)
at com.bulletphysics.demos.opengl.FontRender$GLFont.<init>(FontRender.java:68)
at com.bulletphysics.demos.opengl.LwjglGL.init(LwjglGL.java:55)
at com.bulletphysics.demos.opengl.LWJGL.main(LWJGL.java:60)
at com.bulletphysics.demos.basic.BasicDemo.main(BasicDemo.java:221)

I saw a couple of pages that suggested using an old version of JBullet to avoid this error, but a quick read of the API documentation for java.nio.Buffer made me think this wouldn’t be so hard to fix. Here’s the change I made to jbullet-20101010/src/com/bulletphysics/demos/opengl/FontRender.java:

// next line is the one that causes Exception
// glGenTextures(IntBuffer.wrap(id));
ByteBuffer bb = ByteBuffer.allocateDirect(4);
// ENDIANNESS has no effect here?
// bb.order(ByteOrder.LITTLE_ENDIAN);
IntBuffer ib = bb.asIntBuffer();
ib.put(1);
glGenTextures(ib);

recompiling FontRender.java leads to a similar Exception in com.bulletphysics.demos.opengl.LwjglGL.glLight, so a similar change in jbullet-20101010/src/com/bulletphysics/demos/opengl/LwjglGL.java:

/*
public void glLight(int light, int pname, float[] params) {
  GL11.glLight(light, pname, FloatBuffer.wrap(params));
}
*/
public void glLight(int light, int pname, float[] params) {
  ByteBuffer bb = ByteBuffer.allocateDirect(params.length * 4);
  bb.order(ByteOrder.LITTLE_ENDIAN);
  FloatBuffer fb = bb.asFloatBuffer();
  for (float f : params)
    fb.put(f);
  fb.flip();
  GL11.glLight(light, pname, fb);
}

… and we get the demo! I hope this helps someone. I’m sorry there’s no explanation about that ENDIAN constant. Omitting the line in glLight() produces a working demo, but without light. I’m guessing the explanation for why it’s needed is quite straightforward, but it’s 2:30am and I must sleep.

PS – the code looked horrible, and I realised I’ve been posting horribly-formatted code for donkey’s years, so I installed the WordPress SyntaxHighlighter Evolved plugin. I think it looks pretty good – well, comparatively!