Building a Touch-Responsive OpenGL Triangle on Android

package com.example.opengltest;

import android.app.Activity;
import android.os.Bundle;

public class OpenGLTestActivity extends Activity {

    private MyGLSurfaceView glSurfaceView;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        glSurfaceView = new MyGLSurfaceView(this);
        setContentView(glSurfaceView);
    }
}

package com.example.opengltest;

import android.app.Activity;
import android.opengl.GLSurfaceView;
import android.view.MotionEvent;

public class MyGLSurfaceView extends GLSurfaceView {

    MyRenderer renderer;

    private final float TOUCH_SCALE_FACTOR = 180.0f / 320;
    private float mPreviousX;
    private float mPreviousY;

    public MyGLSurfaceView(Activity aActivity) {
        super(aActivity);

        //OpenGL ES 2.0
        setEGLContextClientVersion(2);

        renderer = new MyRenderer();
        setRenderer(renderer);

        // Render the view only when there is a change in the drawing data
        setRenderMode(GLSurfaceView.RENDERMODE_WHEN_DIRTY);
    }

    @Override
    public boolean onTouchEvent(MotionEvent e) {
        // MotionEvent reports input details from the touch screen
        // and other input controls. In this case, you are only
        // interested in events where the touch position changed.

        float x = e.getX();
        float y = e.getY();

        switch (e.getAction()) {
            case MotionEvent.ACTION_MOVE:

                float dx = x - mPreviousX;
                float dy = y - mPreviousY;

                // reverse direction of rotation above the mid-line
                if (y> getHeight() / 2) {
                    dx = dx * -1 ;
                }

                // reverse direction of rotation to left of the mid-line
                if (x <getWidth() / 2) {
                    dy = dy * -1 ;
                }

                renderer.mAngle += (dx + dy) * TOUCH_SCALE_FACTOR;  // = 180.0f / 320
                requestRender();
        }


        mPreviousX = x;
        mPreviousY = y;
        return true;
    }
}

package com.example.opengltest;

import android.opengl.GLES20;
import android.opengl.GLSurfaceView;
import android.opengl.Matrix;

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

public class MyRenderer implements GLSurfaceView.Renderer {

    Triangle triangle;

    private final float[] projectionMatrix = new float[16];
    private final float[] mMVPMatrix = new float[16];
    private final float[] mVMatrix = new float[16];
    private final float[] mRotationMatrix = new float[16];

    // Declare as volatile because we are updating it from another thread
    public volatile float mAngle;

    @Override
    public void onSurfaceCreated(GL10 gl, EGLConfig config) {
        triangle = new Triangle();
    }

    @Override
    public void onSurfaceChanged(GL10 gl, int width, int height) {
        GLES20.glViewport(0, 0, width, height);

        float ratio = (float) width / height;

        Matrix.frustumM(projectionMatrix, 0, -ratio, ratio, -1, 1, 3, 7);
    }

    @Override
    public void onDrawFrame(GL10 gl) {

        // Set the camera position (View matrix)
        Matrix.setLookAtM(mVMatrix, 0, 0, 0, -3, 0f, 0f, 0f, 0f, 1.0f, 0.0f);

        // Calculate the projection and view transformation
        Matrix.multiplyMM(mMVPMatrix, 0, projectionMatrix, 0, mVMatrix, 0);

        // Create a rotation transformation for the triangle
        // Create a rotation for the triangle
        // long time = SystemClock.uptimeMillis() % 4000L;
        // float angle = 0.090f * ((int) time);
        Matrix.setRotateM(mRotationMatrix, 0, mAngle, 0, 0, -1.0f);

        // Combine the rotation matrix with the projection and camera view
        Matrix.multiplyMM(mMVPMatrix, 0, mRotationMatrix, 0, mMVPMatrix, 0);


        triangle.draw(mMVPMatrix);
    }
}

package com.example.opengltest;

import android.opengl.GLES20;

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

public class Triangle {

    private final String vertexShaderCode =
            // This matrix member variable provides a hook to manipulate
            // the coordinates of the objects that use this vertex shader
            "uniform mat4 uMVPMatrix;" +

            "attribute vec4 vPosition;" +
            "void main() {" +
            // the matrix must be included as a modifier of gl_Position
            "  gl_Position = vPosition * uMVPMatrix;" +
            "}";

    private final String fragmentShaderCode =
            "precision mediump float;" +
            "uniform vec4 vColor;" +
            "void main() {" +
            "  gl_FragColor = vColor;" +
            "}";

    private FloatBuffer vertexBuffer;

    // number of coordinates per vertex in this array
    static final int COORDS_PER_VERTEX = 3;
    static final int vertexStride = COORDS_PER_VERTEX * 4; // 4 bytes per vertex

    static float triangleCoords[] = { // in counterclockwise order:
            0.0f, 0.622008459f, 0.0f,   // top
            -0.5f, -0.311004243f, 0.0f,   // bottom left
            0.5f, -0.311004243f, 0.0f    // bottom right
    };
    static final int vertexCount = triangleCoords.length / COORDS_PER_VERTEX;

    // Set color with red, green, blue and alpha (opacity) values
    float color[] = {0.63671875f, 0.76953125f, 0.22265625f, 1.0f};

    int renderProgram;
    int vPositionHandle;
    int vColorHandle;
    int mvpHandle;

    public Triangle() {
        // initialize vertex byte buffer for shape coordinates
        ByteBuffer bb = ByteBuffer.allocateDirect(
                // (number of coordinate values * 4 bytes per float)
                triangleCoords.length * 4);
        // use the device hardware's native byte order
        bb.order(ByteOrder.nativeOrder());

        // create a floating point buffer from the ByteBuffer
        vertexBuffer = bb.asFloatBuffer();
        // add the coordinates to the FloatBuffer
        vertexBuffer.put(triangleCoords);
        // set the buffer to read the first coordinate
        vertexBuffer.position(0);

        int vertexShader = OpenGLUtils.loadShader(GLES20.GL_VERTEX_SHADER, vertexShaderCode);
        int fragmentShader = OpenGLUtils.loadShader(GLES20.GL_FRAGMENT_SHADER, fragmentShaderCode);

        renderProgram = GLES20.glCreateProgram();             // create empty OpenGL ES Program
        GLES20.glAttachShader(renderProgram, vertexShader);   // add the vertex shader to program
        GLES20.glAttachShader(renderProgram, fragmentShader); // add the fragment shader to program
        GLES20.glLinkProgram(renderProgram);
    }

    public void draw(float[] mvpMatrix) {

        // Add program to OpenGL ES environment
        GLES20.glUseProgram(renderProgram);

        // get handle to vertex shader's vPosition member
        vPositionHandle = GLES20.glGetAttribLocation(renderProgram, "vPosition");

        // Enable a handle to the triangle vertices
        GLES20.glEnableVertexAttribArray(vPositionHandle);

        // Prepare the triangle coordinate data
        GLES20.glVertexAttribPointer(vPositionHandle, COORDS_PER_VERTEX,
                GLES20.GL_FLOAT, false,
                vertexStride, vertexBuffer);

        // get handle to fragment shader's vColor member
        vColorHandle = GLES20.glGetUniformLocation(renderProgram, "vColor");

        // Set color for drawing the triangle
        GLES20.glUniform4fv(vColorHandle, 1, color, 0);

        // get handle to shape's transformation matrix
        mvpHandle = GLES20.glGetUniformLocation(renderProgram, "uMVPMatrix");
        OpenGLUtils.checkGlError("glGetUniformLocation");

        // Apply the projection and view transformation
        GLES20.glUniformMatrix4fv(mvpHandle, 1, false, mvpMatrix, 0);
        OpenGLUtils.checkGlError("glUniformMatrix4fv");

        // Draw the triangle
        GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, vertexCount);

        // Disable vertex array
        GLES20.glDisableVertexAttribArray(vPositionHandle);
    }
}

package com.example.opengltest;

import android.opengl.GLES20;
import android.util.Log;

public class OpenGLUtils {

    public static int loadShader(int type, String shaderCode) {

        // create a vertex shader type (GLES20.GL_VERTEX_SHADER)
        // or a fragment shader type (GLES20.GL_FRAGMENT_SHADER)
        int shader = GLES20.glCreateShader(type);

        // add the source code to the shader and compile it
        GLES20.glShaderSource(shader, shaderCode);
        GLES20.glCompileShader(shader);

        return shader;
    }

    public static void checkGlError(String glOperation) {
        int error;
        while ((error = GLES20.glGetError()) != GLES20.GL_NO_ERROR) {
            Log.e("OpenGLUtils", glOperation + ": glError " + error);
            throw new RuntimeException(glOperation + ": glError " + error);
        }
    }
}