Question

This is probably a very novice question, but I could not find an answer.

Using the simple tutorial from google (http://developer.android.com/resources/tutorials/opengl/opengl-es20.html)

I was trying to draw a square (errm, using common sense?), but getting funny results. It would be grateful if somebody can tell me what is wrong.

   private void initShapes(){    
    float triangleCoords[] = {
        // X, Y, Z
        -0.5f, -0.25f, 0,
         0.5f, -0.25f, 0,
         0.0f,  0.559016994f, 0,
         0.0f,  0f, 0
    }; 

    // initialize vertex Buffer for triangle  
    ByteBuffer vbb = ByteBuffer.allocateDirect(
            // (# of coordinate values * 4 bytes per float)
            triangleCoords.length * 4); 
    vbb.order(ByteOrder.nativeOrder());// use the device hardware's native byte order
    triangleVB = vbb.asFloatBuffer();  // create a floating point buffer from the ByteBuffer
    triangleVB.put(triangleCoords);    // add the coordinates to the FloatBuffer
    triangleVB.position(0);            // set the buffer to read the first coordinate    
}


public void onDrawFrame(GL10 unused) {    
    // Redraw background color
    GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT | GLES20.GL_DEPTH_BUFFER_BIT);

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

    GLES20.glVertexAttribPointer(maPositionHandle, 4, GLES20.GL_FLOAT, false, 12, triangleVB);
    GLES20.glEnableVertexAttribArray(maPositionHandle);
    GLES20.glDrawArrays(GLES20.GL_LINE_LOOP, 0, 4);

}

Everything else same from tutorial.

private final String vertexShaderCode = 
    "attribute vec4 vPosition; \n" +
    "void main(){              \n" +
    " gl_Position = vPosition; \n" +
    "}                         \n";

private final String fragmentShaderCode = 
    "precision mediump float;  \n" +
    "void main(){              \n" +
    " gl_FragColor = vec4 (0.63671875, 0.76953125, 0.22265625, 1.0); \n" +
    "}                         \n";

private int mProgram;
private int maPositionHandle;

private 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 void onSurfaceCreated(GL10 unused, EGLConfig config) {
    initShapes();

    // Set the background frame color
    GLES20.glClearColor(0f, 0f, 0f, 1.0f);

    int vertexShader = loadShader(GLES20.GL_VERTEX_SHADER, vertexShaderCode);
    int fragmentShader = 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);                  // creates OpenGL program executables

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

Thank you!

Answer 1

As I posted in the comments above:

it seems like you might want to change the size parameter from the glVertexAttribPointer call from 4 to 3, since you aren’t using a 4d vector (XYZW).

Cheers