Question

I ran into different behaviors with the GLSurfaceView.
AFAIK it is the responsibility of the program to clear the buffer (color and depth ) each frame. Which means that if I don’t clear the buffer I get the content of the last frame ( or the one before that for double buffering ).

It seems though as if the buffer is cleared no matter what on some devices. I ran the following modification of the “Hello Triangle” program from the Addison Wesley OpenglES2.0 Programming Guide on some test devices with different results:

• Acer Iconia A500 (4.0.3): not cleared (expected behavior)
• Sony XPERIA Go (4.0.4): cleared
• Galaxy S3 (4.1.1): cleared
• LG Optimus 4x HD (4.0.3): not cleared
• Samsung Galaxy Tab 20.1 (4.0.4): not cleared
• Motorola Xoom ( 3.2): not cleared
• Galaxy S2 (4.1.2 – rooted): cleared

Is there a way to force getting an unchanged buffer with each draw callback?

The result for the devices with cleared screen looks like this:

The test activity looks like this:

package com.example.glcleartest;

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

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

import android.opengl.GLES20;
import android.opengl.GLSurfaceView;
import android.opengl.GLSurfaceView.Renderer;
import android.os.Bundle;
import android.app.Activity;
import android.util.Log;

public class MainActivity extends Activity {

protected static final int NUM_VERTICES = 3;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    GLSurfaceView glview = (GLSurfaceView) findViewById(R.id.glview);
    glview.setEGLConfigChooser(false);
    glview.setEGLContextClientVersion(2);
    glview.setRenderer(new Renderer() {

        private int programObject;
        private FloatBuffer vertexBuffer;

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

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

        @Override
        public void onDrawFrame(GL10 gl) {
            float x = 0.1f*(float) Math.sin(System.currentTimeMillis()/1000.0);
            float[] vVertices = new float[]{x, 0.5f, 0.0f,
                    x-0.5f, -0.5f, 0.0f,
                    x+0.5f, -0.5f, 0.0f};
            vertexBuffer.rewind();
            vertexBuffer.put(vVertices);
            vertexBuffer.rewind();


            // Use the program object
            GLES20.glUseProgram(programObject);
            int handle = GLES20.glGetUniformLocation(programObject, "uColor");
            float r = (float) (0.5f+Math.sin(System.currentTimeMillis()/1000.0));
            float g = (float) (0.5f+Math.sin(System.currentTimeMillis()/300.0));
            GLES20.glUniform4f(handle, r, g,0,1);

            // Load the vertex data
            GLES20.glVertexAttribPointer(0, 3, GLES20.GL_FLOAT, false, 0, vertexBuffer);
            GLES20.glEnableVertexAttribArray(0);
            GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, 3);             
        }

        private void error(String s) {
            Log.e("GLTEST", s);
        }

        private int loadShader(int shaderType, String source) {
            if (shaderType != GLES20.GL_FRAGMENT_SHADER && shaderType != GLES20.GL_VERTEX_SHADER) {
                throw new RuntimeException("Illegal shader type");
            }

            int shader = GLES20.glCreateShader(shaderType);
            if (shader != 0) {
                GLES20.glShaderSource(shader, source);
                GLES20.glCompileShader(shader);
                int[] compiled = new int[1];
                GLES20.glGetShaderiv(shader, GLES20.GL_COMPILE_STATUS, compiled, 0);
                if (compiled[0] == 0) {
                    error("Could not compile shader :");
                    error(GLES20.glGetShaderInfoLog(shader));
                    GLES20.glDeleteShader(shader);
                    shader = 0;
                    throw new RuntimeException("Shader Syntax / compilation error");
                }
            }
            return shader;
        }

        private void init() {
            String vShaderStr = "attribute vec4 vPosition; \n" + 
                                    "void main() \n" + "{ \n" + 
                                    " gl_Position = vPosition; \n" + 
                                    "} \n";
            String fShaderStr = "precision mediump float; \n" +
                                        "uniform vec4 uColor;" +
                                        "void main() \n" + 
                                        "{ \n" + 
                                        " gl_FragColor = uColor; \n" + 
                                        "} \n";

            ByteBuffer vbb = ByteBuffer.allocateDirect(NUM_VERTICES*3*4);
            vbb.order(ByteOrder.nativeOrder());
            vertexBuffer = vbb.asFloatBuffer();

            int vertexShader;
            int fragmentShader;

            // Load the vertex/fragment shaders
            vertexShader = loadShader(GLES20.GL_VERTEX_SHADER, vShaderStr);
            fragmentShader = loadShader(GLES20.GL_FRAGMENT_SHADER, fShaderStr);

            // Create the program object
            programObject = GLES20.glCreateProgram();
            if (programObject == 0)
                return;

            GLES20.glAttachShader(programObject, vertexShader);
            GLES20.glAttachShader(programObject, fragmentShader);
            // Bind vPosition to attribute 0
            GLES20.glBindAttribLocation(programObject, 0, "vPosition");
            // Link the program
            GLES20.glLinkProgram(programObject);
            int[] linkStatus = new int[1];
            GLES20.glGetProgramiv(programObject, GLES20.GL_LINK_STATUS, linkStatus, 0);

            if (linkStatus[0] != GLES20.GL_TRUE) {
                error("Could not link program: ");
                error(GLES20.glGetProgramInfoLog(programObject));
                GLES20.glDeleteProgram(programObject);
                programObject = 0;
            }
        }
    });
}
}

Answer 1

If you want to keep your backbuffer contents after swapping, you have to set the EGL_SWAP_BEHAVIOR attribute of your swap surface to EGL_BUFFER_PRESERVED, as documented by the EGL API. Do realize though that on most platforms, this will be a fairly large performance hit. You’re much better off just redrawing the frame in most cases.

For a bit of history: see http://www.khronos.org/registry/egl/specs/EGLTechNote0001.html