Panda3D Manual: How to Control Render Order
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How to Control Render Order In most simple scenes, you can naively attach geometry to the scene graph and let Panda decide the order in which objects should be rendered. Generally, it will do a good enough job, but there are occasions in which it is necessary to step in and take control of the process. To do this well, you need to understand the implications of render order. In a typical OpenGL- or DirectX-style Z-buffered system, the order in which primitives are sent to the graphics hardware is theoretically unimportant, but in practice there are many important reasons for rendering one object before another. Firstly, state sorting is one important optimization. This means choosing to render things that have similar state (texture, color, etc.) all at the same time, to minimize the number of times the graphics hardware has to be told to change state in a particular frame. This sort of optimization is particularly important for very high-end graphics hardware, which achieves its advertised theoretical polygon throughput only in the absence of any state changes; for many such advanced cards, each state change request will completely flush the register cache and force a restart of the pipeline. Secondly, some hardware has a different optimization requirement, and may benefit from drawing nearer things before farther things, so that the Z-buffer algorithm can effectively short-circuit some of the advanced shading features in the graphics card for pixels that would be obscured anyway. This sort of hardware will draw things fastest when the scene is sorted in order from the nearest object to the farthest object, or "front-to-back" ordering. Finally, regardless of the rendering optimizations described above, a particular sorting order is required to render transparency properly (in the absence of the specialized transparency support that only a few graphics cards provide). Transparent and semitransparent objects are normally rendered by blending their semitransparent parts with what has already been drawn to the framebuffer, which means that it is important that everything that will appear behind a semitransparent object must have already been drawn before the semitransparent parts of the occluding object is drawn. This implies that all semitransparent objects must be drawn in order from farthest away to nearest, or in "back-to-front" ordering, and furthermore that the opaque objects should all be drawn before any of the semitransparent objects. Panda achieves these sometimes conflicting sorting requirements through the use of bins. Cull Bins The CullBinManager is a global object that maintains a list of all of the cull bins in the world, and their properties. Initially, there are five default bins, and they will be rendered in the following order:
       Bin Name        Sort  Type
       --------------  ----  ----------------
       "background"     10   BT_fixed
       "opaque"         20   BT_state_sorted
       "transparent"    30   BT_back_to_front
       "fixed"          40   BT_fixed
       "unsorted"       50   BT_unsorted
When Panda traverses the scene graph each frame for rendering, it assigns each Geom it encounters into one of the bins defined in the CullBinManager. (The above lists only the default bins. Additional bins may be created as needed, using either the CullBinManager::add_bin() method, or the Config.prc "cull-bin" variable.) You may assign a node or nodes to an explicit bin using the NodePath::set_bin() interface. set_bin() requires two parameters, the bin name and an integer sort parameter; the sort parameter is only meaningful if the bin type is BT_fixed (more on this below), but it must always be specified regardless. If a node is not explicitly assigned to a particular bin, then Panda will assign it into either the "opaque" or the "transparent" bin, according to whether it has transparency enabled or not. (Note that the reverse is not true: explicitly assigning an object into the "transparent" bin does not automatically enable transparency for the object.) When the entire scene has been traversed and all objects have been assigned to bins, then the bins are rendered in order according to their sort parameter. Within each bin, the contents are sorted according to the bin type. If you want simple geometry that's in back of something to render in front of something that it logically shouldn't, add the following code to the model that you want in front:
    model.setBin("fixed", 40)
    model.setDepthTest(False)
    model.setDepthWrite(False)
The above code will only work for simple models. If your model self occludes (parts of the model covers other parts of the model), the code will not work as expected. An alternative method is to use a display region with `displayRegion.clearDepthActive(True)`. The following bin types may be specified:
     BT_fixed
       Render all of the objects in the bin in a fixed order specified by
       the user.  This is according to the second parameter of the
       NodePath::set_bin() method; objects with a lower value are drawn
       first.
     BT_state_sorted
       Collects together objects that share similar state and renders
       them together, in an attempt to minimize state transitions in the
       scene.
     BT_back_to_front
       Sorts each Geom according to the center of its bounding volume, in
       linear distance from the camera plane, so that farther objects are
       drawn first.  That is, in Panda's default right-handed Z-up
       coordinate system, objects with large positive Y are drawn before
       objects with smaller positive Y.
     BT_front_to_back
       The reverse of back_to_front, this sorts so that nearer objects
       are drawn first.
     BT_unsorted
       Objects are drawn in the order in which they appear in the scene
       graph, in a depth-first traversal from top to bottom and then from
       left to right.
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