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  glDrawArrays() causes JVM to crash  (Read 677 times)
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Offline juhkob

Junior Newbie

« Posted 2013-12-30 02:33:50 »

Hi, I'm using LWJGL. I'm trying to draw 2D shadows using vertex arrays.

//This is the maximum number of shadow vertices there could possibly be. There probably won't be this many.
FloatBuffer shadows = BufferUtils.createFloatBuffer(2 * 4 * 3 * map.entities.size());
//2 coordinates x 4 per shadow quad x 3 (max) shadows per entity x number of entities

I calculate the shadows for each entity and add them to the buffer
shadows.put(new float[] {
      vertex.getX(), vertex.getY(),
      p1.getX(), p1.getY(),
      p2.getX(), p2.getY(),
      nextVertex.getX(), nextVertex.getY()

This is where I actually render the shadows.
glVertexPointer(2, 0, shadows);

glDrawArrays(GL_QUADS, 0, shadows.remaining()); //This is the line that causes the crash


I disable GL_COLOR_ARRAY because there is no color data for the vertices. I don't know if it makes a difference, but I am rendering to the stencil buffer. This shouldn't be an issue because I can do this correctly in immediate mode.

Here's the JVM error that I receive:

Additionally, the shadows seemingly randomly glitch out for a few seconds then return back to normal. The glitching looks like this:

It should look like this:

I can confirm that the math is correct, as this all works flawlessly using immediate mode.

I cannot reliably make it crash. It always crashes, but it does so at seemingly random times. Sometimes it only works for 1 second, sometimes a whole minute.

I have no idea what is happening or what I am doing wrong. Any ideas?

Offline Riven

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Hand over your head.

« Reply #1 - Posted 2013-12-30 10:33:23 »

glDrawArrays(GL_QUADS, 0, shadows.remaining());

The last argument of glDrawArrays is the number of indices to render.
You're passing it the number of floats in your buffer.

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Offline juhkob

Junior Newbie

« Reply #2 - Posted 2013-12-31 06:03:01 »

Thanks! Apologies for the lame question Tongue

I actually visited that page several times and compared the parameters to my code. "Number of indices" sounded like the number of items in the buffer to me, not the number of vertices. Oh well!

glDrawArrays(GL_QUADS, 0, shadows.remaining() / 2);

This appears to work now. Thanks again
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