[AD] Possible issue with the way allegro uses FBOs?

[Thomas Fjellstrom <tfjellstrom@xxxxxxxxxx>]

Somewhat recently the intel-gfx linux drivers received FBO support, and it 
seems to work.

Unfortunately with allegro's ex_bitmap_target example it gets a whopping 
8-10fps, 2 more than the forced bitmap locking.

I found an gl fbo example on the web which seems to render at 50-60fps just 
fine. Here is the actual fbo code it uses:

     // init:
     // create a framebuffer object, you need to delete them when program 
     // exits.
     glGenFramebuffersEXT(1, &fboId);
     glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);

     // create a renderbuffer object to store depth info
     // NOTE: A depth renderable image should be attached the FBO for depth 
     // test. If we don't attach a depth renderable image to the FBO, then 
     // the rendering output will be corrupted because of missing depth 
     // test. If you also need stencil test for your rendering, then you 
     // must attach additional image to the stencil attachement point, too.
     glGenRenderbuffersEXT(1, &rboId);
     glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, rboId);
     glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT, 
        TEXTURE_WIDTH, TEXTURE_HEIGHT);
     glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, 0);

     // attach a texture to FBO color attachement point
     glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, 
        GL_TEXTURE_2D, textureId, 0);

     // attach a renderbuffer to depth attachment point
     glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, 
        GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, rboId);

	// draw:

	// set the rendering destination to FBO
     glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);

    // clear buffer
    glClearColor(1, 1, 1, 1);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    // draw a rotating teapot
    glPushMatrix();
    glRotatef(angle*0.5f, 1, 0, 0);
    glRotatef(angle, 0, 1, 0);
    glRotatef(angle*0.7f, 0, 0, 1);
    glTranslatef(0, -1.575f, 0);
    drawTeapot();
    glPopMatrix();

    // back to normal window-system-provided framebuffer
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0); // unbind

    // trigger mipmaps generation explicitly
    // NOTE: If GL_GENERATE_MIPMAP is set to GL_TRUE, then 			
    // glCopyTexSubImage2D() triggers mipmap generation automatically. 
    // However, the texture attached onto a FBO should generate mipmaps 
    // manually via glGenerateMipmapEXT().
    glBindTexture(GL_TEXTURE_2D, textureId);
    glGenerateMipmapEXT(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, 0);

    ....

    // draw rotating cube here


Is there something allegro is or isn't doing that might explain the 
difference? If there is something we're doing wrong, it could improve 
performance on a /lot/ of integrated intel gpus.

-- 
Thomas Fjellstrom
tfjellstrom@xxxxxxxxxx