Android openGL example gives blank screen

Question

I just tried the original open GL example code from developer.android.com on a galaxy s3 and all It gave me was a blank screen! I changed the clear color and the screen background changed but I still can't see any triangle or square as the code indicates.

To verify any problem with GPU I ran some compiled apps from play store and all run fine then I tried with my own gl code following the tutorial and again It gave a blank screen then I tried a third party example code and guess what again blank screen!

What the hell is wrong the GL doesn't seem to draw anything except background color

Here's a link to android example code I used

http://developer.android.com/shareables/training/OpenGLES.zip

This is the code

 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.example.android.opengl;

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

import android.opengl.GLES20;

/**
 * A two-dimensional triangle for use as a drawn object in OpenGL ES 2.0.
 */
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
            // Note that the uMVPMatrix factor *must be first* in order
            // for the matrix multiplication product to be correct.
            "  gl_Position = uMVPMatrix * vPosition;" +
            "}";

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

    private final FloatBuffer vertexBuffer;
    private final int mProgram;
    private int mPositionHandle;
    private int mColorHandle;
    private int mMVPMatrixHandle;

    // number of coordinates per vertex in this array
    static final int COORDS_PER_VERTEX = 3;
    static float triangleCoords[] = {
            // in counterclockwise order:
            5.0f,  0.622008459f, 0.0f,   // top
           -5.5f, -0.311004243f, 0.0f,   // bottom left
            5.5f, -0.311004243f, 0.0f    // bottom right
    };
    private final int vertexCount = triangleCoords.length / COORDS_PER_VERTEX;
    private final int vertexStride = COORDS_PER_VERTEX * 4; // 4 bytes per vertex

    float color[] = { 0.63671875f, 0.76953125f, 0.22265625f, 0.0f };

    /**
     * Sets up the drawing object data for use in an OpenGL ES context.
     */
    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);

        // prepare shaders and OpenGL program
        int vertexShader = MyGLRenderer.loadShader(
                GLES20.GL_VERTEX_SHADER, vertexShaderCode);
        int fragmentShader = MyGLRenderer.loadShader(
                GLES20.GL_FRAGMENT_SHADER, fragmentShaderCode);

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

    }

    /**
     * Encapsulates the OpenGL ES instructions for drawing this shape.
     *
     * @param mvpMatrix - The Model View Project matrix in which to draw
     * this shape.
     */
    public void draw(float[] mvpMatrix) {
        // Add program to OpenGL environment
        GLES20.glUseProgram(mProgram);

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

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

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

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

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

        // get handle to shape's transformation matrix
        mMVPMatrixHandle = GLES20.glGetUniformLocation(mProgram, "uMVPMatrix");
        MyGLRenderer.checkGlError("glGetUniformLocation");

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

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

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

}




/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.example.android.opengl;

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

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

/**
 * Provides drawing instructions for a GLSurfaceView object. This class
 * must override the OpenGL ES drawing lifecycle methods:
 * <ul>
 *   <li>{@link android.opengl.GLSurfaceView.Renderer#onSurfaceCreated}</li>
 *   <li>{@link android.opengl.GLSurfaceView.Renderer#onDrawFrame}</li>
 *   <li>{@link android.opengl.GLSurfaceView.Renderer#onSurfaceChanged}</li>
 * </ul>
 */
public class MyGLRenderer implements GLSurfaceView.Renderer {

    private static final String TAG = "MyGLRenderer";
    private Triangle mTriangle;
    private Square   mSquare;

    // mMVPMatrix is an abbreviation for "Model View Projection Matrix"
    private final float[] mMVPMatrix = new float[16];
    private final float[] mProjectionMatrix = new float[16];
    private final float[] mViewMatrix = new float[16];
    private final float[] mRotationMatrix = new float[16];

    private float mAngle;

    @Override
    public void onSurfaceCreated(GL10 unused, EGLConfig config) {

        // Set the background frame color
        GLES20.glClearColor(0.0f, 0.0f, 0.2f, 1.0f);

        mTriangle = new Triangle();
        mSquare   = new Square();
    }

    @Override
    public void onDrawFrame(GL10 unused) {
        float[] scratch = new float[16];

        // Draw background color
        GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT | GLES20.GL_DEPTH_BUFFER_BIT);

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

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

        // Draw square
        mSquare.draw(mMVPMatrix);

        // Create a rotation for the triangle

        // Use the following code to generate constant rotation.
        // Leave this code out when using TouchEvents.
        // 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
        // Note that the mMVPMatrix factor *must be first* in order
        // for the matrix multiplication product to be correct.
        Matrix.multiplyMM(scratch, 0, mMVPMatrix, 0, mRotationMatrix, 0);

        // Draw triangle
        mTriangle.draw(scratch);
    }

    @Override
    public void onSurfaceChanged(GL10 unused, int width, int height) {
        // Adjust the viewport based on geometry changes,
        // such as screen rotation
        GLES20.glViewport(0, 0, width, height);

        float ratio = (float) width / height;

        // this projection matrix is applied to object coordinates
        // in the onDrawFrame() method
        Matrix.frustumM(mProjectionMatrix, 0, -ratio, ratio, -1, 1, 3, 7);

    }

    /**
     * Utility method for compiling a OpenGL shader.
     *
     * <p><strong>Note:</strong> When developing shaders, use the checkGlError()
     * method to debug shader coding errors.</p>
     *
     * @param type - Vertex or fragment shader type.
     * @param shaderCode - String containing the shader code.
     * @return - Returns an id for the shader.
     */
    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;
    }

    /**
    * Utility method for debugging OpenGL calls. Provide the name of the call
    * just after making it:
    *
    * <pre>
    * mColorHandle = GLES20.glGetUniformLocation(mProgram, "vColor");
    * MyGLRenderer.checkGlError("glGetUniformLocation");</pre>
    *
    * If the operation is not successful, the check throws an error.
    *
    * @param glOperation - Name of the OpenGL call to check.
    */
    public static void checkGlError(String glOperation) {
        int error;
        while ((error = GLES20.glGetError()) != GLES20.GL_NO_ERROR) {
            Log.e(TAG, glOperation + ": glError " + error);
            throw new RuntimeException(glOperation + ": glError " + error);
        }
    }

    /**
     * Returns the rotation angle of the triangle shape (mTriangle).
     *
     * @return - A float representing the rotation angle.
     */
    public float getAngle() {
        return mAngle;
    }

    /**
     * Sets the rotation angle of the triangle shape (mTriangle).
     */
    public void setAngle(float angle) {
        mAngle = angle;
    }

}

Answer 1

for me, the fix came from comparing these two matrices

1. one sets up the view (the eye) to be "-3" in Z-axis:

   Matrix.setLookAtM(mViewMatrix, 0, 0, 0, -3, 0f, 0f, 0f, 0f, 1.0f, 0.0f);

2. the other sets up the space that should be visible. in the original google sample, the "near" plane is set to 3, and the "far" plane to 7:

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

this puts your triangle on the very edge of the displayed space. it seems that these intervals are open, as being on the very edge does not display anything.

to adjust this, i simply changed the definition of displayed space to "near = 2" as follows: Matrix.frustumM(mProjectionMatrix, 0, -ratio, ratio, -1, 1, 2, 7);

and boom, suddenly i was in business ;-)

i'm not sure why the original google sample is set up such that you actually see nothing. it might be that they put a challenge in to teach us (very early) beginners ;-) hope this helps