I am trying to write a plugin for Unity that does the following on iOS using OpenGL ES 2.0:

1. Read camera image
2. Draw the camera image into an FBO
3. FBO is rendered to a Unity texture.
4. Do stuff with texture in Unity.

The plugin is called in the update function of a script attached to the main camera.

The sticking point comes when I try and draw the textured quad in the offscreen FBO with glDrawArrays(GL_TRIANGLE_STRIP, 0, 4). If I comment out this line the app runs, my 3d scene is drawn correctly and runs at full framerate. All that is missing is the texture. If I enable the line, nothing draws and everything runs at about 3fps. No errors are reported.

This is the complete draw function:

- (void) drawVideoFrame
{
  GLint nCurrentProg;
  glGetIntegerv(GL_CURRENT_PROGRAM, &nCurrentProg);

  GLint nCurrentFrameBuffer;
  GLint nCurrentRenderBuffer;
  glGetIntegerv(GL_FRAMEBUFFER_BINDING, &nCurrentFrameBuffer );
  glGetIntegerv(GL_RENDERBUFFER_BINDING, &nCurrentRenderBuffer );

  glBindFramebuffer(GL_FRAMEBUFFER, nFBOBuffer);
  glBindRenderbuffer(GL_RENDERBUFFER, nFBORender);

  glViewport(0, 0, videoWidth, videoHeight);

  static const GLfloat squareVertices[] = {
     -1.0f, -1.0f,
      1.0f, -1.0f,
      -1.0f,  1.0f,
      1.0f,  1.0f,
    };

    static const GLfloat textureVertices[] = {
      0.0f, 1.0f,
      1.0f, 1.0f,
      0.0f, 0.0f,
      1.0f, 0.0f,
    };

  // Use shader program.
  glUseProgram(passThroughProgram);

  glBindTexture(GL_TEXTURE_2D, nTexID);

  // Update attribute values.
  glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, 0, 0, squareVertices);
  glEnableVertexAttribArray(ATTRIB_VERTEX);
  glVertexAttribPointer(ATTRIB_TEXTUREPOSITON, 2, GL_FLOAT, 0, 0, textureVertices);
  glEnableVertexAttribArray(ATTRIB_TEXTUREPOSITON);

  glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

  glUseProgram(nCurrentProg);
  glBindTexture(GL_TEXTURE_2D, 0);
  glBindFramebuffer(GL_FRAMEBUFFER, nCurrentFrameBuffer);
  glBindRenderbuffer(GL_RENDERBUFFER, nCurrentRenderBuffer);
}

The fragment and vertex programs are just simple pass throughs:

Fragment

varying highp vec2 coordinate;
uniform sampler2D videoframe;

void main()
{
    gl_FragColor = texture2D(videoframe, coordinate);
}

Vertex

attribute vec4 position;
attribute mediump vec4 textureCoordinate;
varying mediump vec2 coordinate;

void main()
{
    gl_Position = position;
    coordinate = textureCoordinate.xy;
}

I took my plugin code and implemented it as a stand alone app, doing the same thing, and everything worked.

Note:

• I am aware that this example is slightly pointless, but it is a minimal example
• I can read the camera image and upload it directly to a unity owned texture, but that is not what I want to do here.

Does anyone have any idea what is wrong?