Question

I’m building an Android app with OpenGL.I created 2 squares, each with their own textures(PNG), and overlayed them. From hints i got from a previous question, i used gl.glBlendFunc(GL10.GL_SRC_ALPHA, GL10.GL_ONE_MINUS_SRC_ALPHA);

My problem is that the transparency effect, affects the second square, therefor i can see the background through the second square’s texture.Is there a way arround this?

Here is the Renderer and at the Bottom the Square.java class :

package hello.project;

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

import javax.microedition.khronos.opengles.GL10;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.opengl.GLUtils;

public class Square {

 private FloatBuffer vertexBuffer; // buffer holding the vertices
 static int sex=R.drawable.girl;

 private FloatBuffer textureBuffer; // buffer holding the texture coordinates
 private float texture[] = {
   // Mapping coordinates for the vertices
   0.0f, 1.0f,  // top left  (V2)
   0.0f, 0.0f,  // bottom left (V1)
   1.0f, 1.0f,  // top right (V4)
   1.0f, 0.0f  // bottom right (V3)
 };

 private float vertices[] = {
   -1.0f, -2.0f,  0.0f,  // V1 - bottom left
   -1.0f,  2.0f,  0.0f,  // V2 - top left
    0.8f, -2.0f,  0.0f,  // V3 - bottom right
    0.8f,  2.0f,  0.0f   // V4 - top right
 };

 public Square() {
  ByteBuffer byteBuffer = ByteBuffer.allocateDirect(vertices.length * 4);
  byteBuffer.order(ByteOrder.nativeOrder());
  vertexBuffer = byteBuffer.asFloatBuffer();
  vertexBuffer.put(vertices);
  vertexBuffer.position(0);

  byteBuffer = ByteBuffer.allocateDirect(texture.length * 4);
  byteBuffer.order(ByteOrder.nativeOrder());
  textureBuffer = byteBuffer.asFloatBuffer();
  textureBuffer.put(texture);
  textureBuffer.position(0);
 }

 /** The draw method for the square with the GL context */
 public void draw(GL10 gl) {
  // bind the previously generated texture
  gl.glBindTexture(GL10.GL_TEXTURE_2D, textures[0]);

  // Point to our buffers
  gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
  gl.glEnableClientState(GL10.GL_TEXTURE_COORD_ARRAY);



  // Set the face rotation
  gl.glFrontFace(GL10.GL_CW);

  // Point to our vertex buffer
  gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertexBuffer);
  gl.glTexCoordPointer(2, GL10.GL_FLOAT, 0, textureBuffer);

  // Draw the vertices as triangle strip
  gl.glDrawArrays(GL10.GL_TRIANGLE_STRIP, 0, vertices.length / 3);

  //Disable the client state before leaving
  gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);
  gl.glDisableClientState(GL10.GL_TEXTURE_COORD_ARRAY);
 }




 /** The texture pointer */
 private int[] textures = new int[1];

 public void loadGLTexture(GL10 gl, Context context,int sex ) {
  // loading texture
  Bitmap bitmap = BitmapFactory.decodeResource(context.getResources(),
    sex);

  // generate one texture pointer
  gl.glGenTextures(1, textures, 0);
  // ...and bind it to our array
  gl.glBindTexture(GL10.GL_TEXTURE_2D, textures[0]);

  // create nearest filtered texture
  gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_MIN_FILTER, GL10.GL_NEAREST);
  gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_MAG_FILTER, GL10.GL_LINEAR);

  // Use Android GLUtils to specify a two-dimensional texture image from our bitmap
  GLUtils.texImage2D(GL10.GL_TEXTURE_2D, 0, bitmap, 0);
  // Clean up
  bitmap.recycle();
 }

 public static int getSex() {
  return sex;
 }

 public static void setSex(int sex) {
  Square.sex = sex;
 }
}
---------------------------------------------------------------------------
package hello.project;

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

import android.content.Context;
import android.opengl.GLSurfaceView.Renderer;
import android.opengl.GLU;

public class HelloOpenGLES10Renderer implements Renderer {

 private Square   square;  // the square
 private Square2  square2;  // the square
 private Context     context;


 /** Constructor to set the handed over context */
 public HelloOpenGLES10Renderer(Context context) {
  this.square  = new Square();
  this.square2 = new Square2();
  this.context=context;
 }

 public void onDrawFrame(GL10 gl) {
  // clear Screen and Depth Buffer
  gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);

  // Reset the Modelview Matrix
  gl.glLoadIdentity();

  // Drawing
  gl.glTranslatef(0.0f, 0.0f, -5.0f);  // move 5 units INTO the screen
  square.draw(gl);
  square2.draw(gl); 


 }


 public void onSurfaceChanged(GL10 gl, int width, int height) {
  if(height == 0) {       //Prevent A Divide By Zero By
   height = 1;       //Making Height Equal One
  }

  gl.glViewport(0, 0, width, height);  //Reset The Current Viewport
  gl.glMatrixMode(GL10.GL_PROJECTION);  //Select The Projection Matrix
  gl.glLoadIdentity();      //Reset The Projection Matrix

  //Calculate The Aspect Ratio Of The Window
  GLU.gluPerspective(gl, 45.0f, (float)width / (float)height, 0.1f, 100.0f);

  gl.glMatrixMode(GL10.GL_MODELVIEW);  //Select The Modelview Matrix
  gl.glLoadIdentity();      //Reset The Modelview Matrix
 }


 public void onSurfaceCreated(GL10 gl, EGLConfig config) {
  // Load the texture for the square

  square.loadGLTexture(gl, this.context,Square.getSex());
  square2.loadGLTexture(gl, this.context,Square2.getSex());

  gl.glEnable(GL10.GL_TEXTURE_2D); 
  gl.glDisable(GL10.GL_DEPTH_TEST);
  gl.glEnable(GL10.GL_BLEND);

  gl.glBlendFunc(GL10.GL_SRC_ALPHA, GL10.GL_ONE_MINUS_SRC_ALPHA);

  gl.glShadeModel(GL10.GL_SMOOTH);    //Enable Smooth Shading
  gl.glClearColor(0.0f, 0.0f, 0.0f, 0.0f);  //Black Background
  gl.glClearDepthf(1.0f);      //Depth Buffer Setup
  gl.glDepthFunc(GL10.GL_NEVER);    //The Type Of Depth Testing To Do

  //Really Nice Perspective Calculations
  gl.glHint(GL10.GL_PERSPECTIVE_CORRECTION_HINT, GL10.GL_NICEST);
 }
}

Answer 1

Don’t you just have a problem with the graphics pipeline state? Remember whichever square you tell to get drawn first gets drawn using the blend function that’s currently active (and this will remain active until you change the OpenGL state). Maybe you want to add some more state changes to the blend function, or change the order of drawing to get the effect you want? You might want to also try enabling/disabling the depth test between drawing calls to make a square opaque.

Hope this helps a bit, you might need to provide a bit more detail to your question.