android OpenGl How to draw a rectangle

Issue

I want to draw a rectangle using OpenGL

package jnidemo.example.com.openglsquare;

import android.content.Context;
import android.opengl.GLSurfaceView;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;

import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.FloatBuffer;
import java.nio.ShortBuffer;

import javax.microedition.khronos.egl.EGLConfig;
import javax.microedition.khronos.opengles.GL10;


public class MainActivity extends ActionBarActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        MySurfaceView mySurfaceView = new MySurfaceView(this);
        setContentView(mySurfaceView);
    }

    class MySurfaceView extends GLSurfaceView {
        public MySurfaceView(Context context) {
            super(context);
            setRenderer(new MyRenderer());
        }
    }

    class MyRenderer implements GLSurfaceView.Renderer {
        @Override
        public void onSurfaceCreated(GL10 gl, EGLConfig config) {
            gl.glClearColor(1.0f, 1.0f, 0.0f, 0.0f);
        }

        @Override
        public void onSurfaceChanged(GL10 gl, int width, int height) {
            gl.glViewport(0, 0, width, height);
            float aspect = (float)width / height;
            gl.glMatrixMode(GL10.GL_PROJECTION);
            gl.glLoadIdentity();
            gl.glFrustumf(-aspect, aspect, -1.0f, 1.0f, 1.0f, 10.0f);
        }

        @Override
        public void onDrawFrame(GL10 gl) {
            gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);

            Square square = new Square();
            square.draw(gl);
        }
    }
}

class Square{
    FloatBuffer vertexbuffer;
    ByteBuffer indicesBuffer;

    float vetices[] = {
        //-0.5f,0.5f,0.0f,
        //0.5f,0.5f,0.0f,
        //0.5f,-0.5f,0.0f,
        //-0.5f,0.5f,0.0f

        -1.0f, -1.0f,
        1.0f, -1.0f,
        -1.0f, 1.0f,
        1.0f, 1.0f
    };

    byte indices[] = {0,1,2,2,1,3};

    public Square(){

        ByteBuffer byteBuffer=ByteBuffer.allocateDirect(vetices.length*4);
        byteBuffer.order(ByteOrder.nativeOrder());
        vertexbuffer = byteBuffer.asFloatBuffer();
        vertexbuffer.put(vetices);
        vertexbuffer.position(0);

        indicesBuffer = ByteBuffer.allocateDirect(indices.length);
        //indicesBuffer.order(ByteOrder.nativeOrder());
        indicesBuffer.put(indices);
        indicesBuffer.position(0);
    }

    public void draw(GL10 gl){
        gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
        gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertexbuffer);

        gl.glDrawElements(GL10.GL_TRIANGLES, indices.length, GL10.GL_UNSIGNED_BYTE, indicesBuffer);

        gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);
    }
}

But nothing appears on the screen.
I don’t want to use any color. I just want to draw a simple square or rectangle so that I can get some idea of simple OpenGL ES drawing mechanisms.

I don’t know what the problem is.

Solution

public void setVerticesAndDraw(Float value, GL10 gl, byte color) {
    FloatBuffer vertexbuffer;
    ByteBuffer indicesBuffer;
    ByteBuffer mColorBuffer;

    byte indices[] = {0, 1, 2, 0, 2, 3};

    float vetices[] = {
        -value, value, 0.0f,
        value, value, 0.0f,
        value, -value, 0.0f,
        -value, -value, 0.0f
    };

    byte colors[] = {
        color, color, 0, color,
        0, color, color, color,
        0, 0, 0, color,
        color, 0, color, maxColor
    };

    ByteBuffer byteBuffer = ByteBuffer.allocateDirect(vetices.length * 4);
    byteBuffer.order(ByteOrder.nativeOrder());
    vertexbuffer = byteBuffer.asFloatBuffer();
    vertexbuffer.put(vetices);
    vertexbuffer.position(0);

    indicesBuffer = ByteBuffer.allocateDirect(indices.length);
    indicesBuffer.put(indices);
    indicesBuffer.position(0);

    mColorBuffer = ByteBuffer.allocateDirect(colors.length);
    mColorBuffer.put(colors);
    mColorBuffer.position(0);

    gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
    gl.glEnableClientState(GL10.GL_COLOR_ARRAY);

    gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertexbuffer);
    gl.glColorPointer(4, GL10.GL_UNSIGNED_BYTE, 0, mColorBuffer);

    gl.glDrawElements(GL10.GL_TRIANGLES, indices.length, GL10.GL_UNSIGNED_BYTE, indicesBuffer);
    gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);
}

And you can call this method like this:

Square square = new Square();
//square.draw(gl);

square.setVerticesAndDraw(0.8f, gl, (byte) 255);

square.setVerticesAndDraw(0.7f, gl, (byte) 150);
square.setVerticesAndDraw(0.6f, gl, (byte) 100);
square.setVerticesAndDraw(0.5f, gl, (byte) 80);
square.setVerticesAndDraw(0.4f, gl, (byte) 50);

You will see a cool image like this:

enter image description here

Answered By – RajSharma

This Answer collected from stackoverflow, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0

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