question about switching shader programs (ES 2)

Question

Hello guys,

I'm trying to write a rather simple opengl es 2 program.

I have two different shaders, one renders a solid color rectangle to the screen, the other one renders a texture mapped rectangle.

I want to first draw the solid color rectangle, and then on top of it, draw the texture mapped rectangle.

    glUseProgram(program1);
  glUniform1f(shader1_uniforms[UNIFORM_SCREEN_WIDTH], (GLfloat)screenWidth);
  glUniform1f(shader1_uniforms[UNIFORM_SCREEN_HEIGHT], (GLfloat)screenHeight);

  glDisableVertexAttribArray(ATTRIB_TEX2);

  glVertexAttribPointer(ATTRIB_VERTEX2, 2, GL_FLOAT, 0, 0, squareVertices);
  glEnableVertexAttribArray(ATTRIB_VERTEX2);
  glVertexAttribPointer(ATTRIB_COLOR2, 4, GL_FLOAT, 1, 0, squareColors);
  glEnableVertexAttribArray(ATTRIB_COLOR2);


  glDrawArrays(GL_TRIANGLE_STRIP,0,4);


   glEnable(GL_BLEND);
   glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

   glBindTexture(GL_TEXTURE_2D,1);




  glUseProgram(program2);
  glUniform1f(shader2_uniforms[UNIFORM_SCREEN_WIDTH], (GLfloat)screenWidth );
  glUniform1f(shader2_uniforms[UNIFORM_SCREEN_HEIGHT], (GLfloat)screenHeight);
  glUniform1i(shader2_uniforms[UNIFORM_UIMAP], 0);



  glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, 0, 0, squareVertices);
  glEnableVertexAttribArray(ATTRIB_VERTEX);
  glVertexAttribPointer(ATTRIB_COLOR, 4, GL_FLOAT, 1, 0, squareColors);
  glEnableVertexAttribArray(ATTRIB_COLOR);
  glVertexAttribPointer(ATTRIB_TEX, 2, GL_FLOAT, 0, 0, squareTexCoords);
  glEnableVertexAttribArray(ATTRIB_TEX);

  // Draw
  glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);


   glDisable(GL_BLEND);

the two shaders work correctly, if executed separately (for example, comment the part using the first shader). but when i combine them together, like the above, the texture map will disappear. I spent quite awhile trying to figure out why...

can someone help me? thanks.

Answer 1

You can't combine shaders that way, only the last program specified with glUseProgram will be used.

To combine shaders, you must do it manually generating a new shader which does both things at the same time.

Answer 2

I wonder if the glBindTexture() must happen after the program is switched with glUseProgram(). Here's a reference for glUseProgram this page. Make sure glUseProgram is not being called between glBegin and glEnd. Check glGetError to see if an error is caused when loading the second vertex program.