Articles - Drawing Simple Primitives with OpenGL ES I [en]

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Drawing Simple Primitives with OpenGL ES I [en]

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Drawing Simple Primitives with OpenGL ES

Our first application with OpenGL ES was a completely blue screen (nothing special), but now we will start drawing shapes on the screen.
To draw any shape with OpenGL ES you need a vertex buffer and an index buffer (you remember these words?). The first thing we’ll do is to create a four vertex buffer and also a four index buffer, this eventually mean four vertices and four indices.

In the MyGLRenderer.java file add the line:

Code JAVA :

import java.nio.*;

This will allow us to use the different buffers.

The following thing to is to add a vertex and index buffer the class as attributes. As you know a vertex is composed from two real numbers in 2D, so the vertex buffer is an array of real numbers, and the index is just a small unsigned integer. So we will use FloatBuffer for vertex buffers and ShortBuffer for index buffer.
Add: FloatBuffer _vertexBuffer; and ShortBuffer _indexBuffer; as protected attributes of the MyGLRenderer class.
Change the onSurfaceCreated code to the following:

Code JAVA :

public void onSurfaceCreated(GL10 gl, EGLConfig config)
	{
	    gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
	    gl.glLineWidth(2);
 
	    ByteBuffer vbb = ByteBuffer.allocateDirect(4 * Float.SIZE * 2);
	    vbb.order(ByteOrder.nativeOrder());
	    _vertexBuffer = vbb.asFloatBuffer();
 
	    float[] fv = { 50, 50,
	    		      -50, 50,
	    		      -50, -50,
	    		      50, -50 };
	    _vertexBuffer.put(fv);
 
	    vbb = ByteBuffer.allocateDirect(4 * Short.SIZE);
	    vbb.order(ByteOrder.nativeOrder());
	    _indexBuffer = vbb.asShortBuffer();
 
	    short[] sv = { 0, 1, 2, 3 };
	    _indexBuffer.put(sv);
 
	    _vertexBuffer.position(0);
	    _indexBuffer.position(0);
 
	}

glEnableClientState(GL10.GL_VERTEX_ARRAY): Enables the vertex array rendering feature.
glLineWidth(2): Change the line width to 2.
We create allocate a new buffer with the size of (4 * Float.SIZE * 2) four vertices, every vertex is two float.
We create an array of floating values representing four vertices forming a square, and put these values into the buffer.
We create an array of integers representing four indices and put them into the buffer.

Now, go to onDrawFrame and add following lines:

Code JAVA :

	     // set the drawing colour
	     gl.glColor4f(1.0f, 0.0f, 0.0f, 1.0f);
 
	     gl.glVertexPointer(2, GL10.GL_FLOAT, 0, _vertexBuffer);
	     gl.glDrawElements(GL10.GL_LINES, 4, GL10.GL_UNSIGNED_SHORT, _indexBuffer);

glColor4f: sets the drawing colour. Here, we set the red colour (red, green, blue, alpha).
glVertexPointer: indicates the vertex buffer to draw.
glDrawElements: draw the vertex buffer using the GL_LINES mode and the given index buffer.

Running the application will produce the following screen:

Ecrit par: OSasuke, Le: 10/05/12

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