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OpenGL DrawArrays acting strangely

General and Gameplay Programming Programming

Started by SHOKWAV March 24, 2012 03:49 PM

4 comments, last by Trienco 12 years, 1 month ago

SHOKWAV

138

Author

March 24, 2012 03:49 PM

Hello. Whenever I try to draw a square using OpenGL's drawarrays function, it acts strange... it draws my square, but it also draws a giant polygon-like thing right below it. I can't get an image (because Vista doesn't capture what I draw with OpenGL with PrntScreen

), but I was wondering if it was something I am doing wrong?

[size="2"]How I call it: drawSquare(30, 30, 20, 20);

Function:

[size=2][color=#7f0055][size=2][color=#7f0055]void[size=2][color=#000000] drawSquare([size=2][color=#7f0055][size=2][color=#7f0055]const[size=2][color=#000000] [size=2][color=#005032][size=2][color=#005032]GLshort[size=2][color=#000000] posX, [size=2][color=#7f0055][size=2][color=#7f0055]const[size=2][color=#000000] [size=2][color=#005032][size=2][color=#005032]GLshort[size=2][color=#000000] posY, [size=2][color=#7f0055][size=2][color=#7f0055]const[size=2][color=#000000] [size=2][color=#005032][size=2][color=#005032]GLshort[size=2][color=#000000] sizeX, [size=2][color=#7f0055][size=2][color=#7f0055]const[size=2][color=#000000] [size=2][color=#005032][size=2][color=#005032]GLshort[size=2][color=#000000] sizeY){

[size=2][color=#005032][size=2][color=#005032]GLshort[size=2] vertices[8];

[size=2][color=#3f7f5f][size=2][color=#3f7f5f]//bottom left

[size=2]vertices[0] = posX;

[size=2]vertices[1] = posY;

[size=2][color=#3f7f5f][size=2][color=#3f7f5f]//bottom right

[size=2]vertices[2] = posX + sizeX;

[size=2]vertices[3] = posY;

[size=2][color=#3f7f5f][size=2][color=#3f7f5f]//top right

[size=2]vertices[4] = posX + sizeX;

[size=2]vertices[5] = posY - sizeY;

[size=2][color=#3f7f5f][size=2][color=#3f7f5f]//top left

[size=2]vertices[6] = posX;

[size=2]vertices[7] = posY - sizeY;

[size=2][color=#642880][size=2][color=#642880]glEnableClientState[size=2](GL_VERTEX_ARRAY);

[size=2][color=#642880][size=2][color=#642880]glEnableClientState[size=2](GL_TEXTURE_COORD_ARRAY);

[size=2][color=#642880][size=2][color=#642880]glVertexPointer[size=2](2, GL_SHORT, 0, vertices);

[size=2][color=#642880][size=2][color=#642880]glColor4f[size=2](1.0f, 1.0f, 0.0f, 0.5f);

[size=2][color=#642880][size=2][color=#642880]glDrawArrays[size=2](GL_QUADS, 0, 8);

[size=2][color=#642880][size=2][color=#642880]glDisableClientState[size=2](GL_VERTEX_ARRAY);

[size=2][color=#642880][size=2][color=#642880]glDisableClientState[size=2](GL_TEXTURE_COORD_ARRAY);

[size=2]}

Trienco

2,555

March 24, 2012 04:27 PM

A (x or y) coordinate isn't a vertex and a quad only has 4 vertices. Why are you passing 8?

f@dzhttp://festini.device-zero.de

SHOKWAV

138

Author

March 24, 2012 04:35 PM

A (x or y) coordinate isn't a vertex and a quad only has 4 vertices. Why are you passing 8?

I thought it treated every two ints as a vertex?

ReaperSMS

1,546

March 24, 2012 05:41 PM

You're already telling it to use 2 shorts per vertex, glDrawArrays expects a vertex count, not a component count.

SHOKWAV

138

Author

March 24, 2012 08:25 PM

You're already telling it to use 2 shorts per vertex, glDrawArrays expects a vertex count, not a component count.

Thanks, guys! Fixed it, and it runs perfectly now. I do have one last question: can I just enable and disable the vertex and textrue arrays when I startup and shutdown my program, or should I enable and disable them every time I draw? I'm trying to conserve speed.

Trienco

2,555

March 25, 2012 07:01 AM

Now that you mention it. You enable texture coord arrays but never set up the pointer. Ignoring what any defaults maybe you are technically lucky that it doesn't crash. In other words, no, it's not safe to always keep them enabled unless you make sure to always have them setup and point at valid data. Deciding "I don't need textures for this part" and just keeping it enabled will use whatever you set up last. And if whatever you set up doesn't have at least as many texture coordinates as you have vertices now, you will be again lucky if it doesn't crash.

It's perfectly fine to have them enabled by default, but not when you don't actually use them.

f@dzhttp://festini.device-zero.de

OpenGL DrawArrays acting strangely

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