title: Adding a decal using multitexturing on an iPhone
body: <p>I'm trying to overlay one image on top of another onto a simple quad. I set my bottom image as texture unit 0, and then my top image (which has a variable alpha) as texture unit 1. Unit 2 has mode <code>GL_DECAL</code>, which means the bottom texture should show up when the alpha is 0, and the top texture should show when the alpha is 1. But, only the top texture shows up and the bottom one doesn't appear at all. It's just white where the bottom texture should show through.</p>
  
  <p>glGetError() doesn't report any problems. Any help is appreciated. Thanks!</p>
  
  <pre><code>glVertexPointer(3, GL_FLOAT, 0, boxVertices);
  glEnableClientState(GL_VERTEX_ARRAY);
  
  glClientActiveTexture(GL_TEXTURE0);
  glEnableClientState(GL_TEXTURE_COORD_ARRAY);
  glTexCoordPointer(2, GL_FLOAT, 0, boxTextureCoords);
  glClientActiveTexture(GL_TEXTURE1);
  glEnableClientState(GL_TEXTURE_COORD_ARRAY);
  glTexCoordPointer(2, GL_FLOAT, 0, boxTextureCoords);
  
  glClientActiveTexture(GL_TEXTURE0);
  glEnable(GL_TEXTURE_2D);
  glClientActiveTexture(GL_TEXTURE1);
  glEnable(GL_TEXTURE_2D);
  
  glClientActiveTexture(GL_TEXTURE0);
  glBindTexture(GL_TEXTURE_2D, one.texture);
  glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
  
  glClientActiveTexture(GL_TEXTURE1);
  glBindTexture(GL_TEXTURE_2D, two.texture);
  glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);
  
  glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
  </code></pre>