Transparency and Blending

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Dealing with Depth-Sorting


Usually transparency works as expected in Panda automatically, but sometimes it just seems to go awry, where a semitransparent object in the background seems to partially obscure a semitransparent object in front of it. This is especially likely to happen with large flat polygon cutouts, or when a transparent object is contained within another transparent object, or when parts of a transparent object can be seen behind other parts of the same object.

The fundamental problem is that correct transparency, in the absence of special hardware support involving extra framebuffer bits, requires drawing everything in order from farthest away to nearest. This means sorting each polygon--actually, each pixel, for true correctness--into back-to-front order before drawing the scene.

It is, of course, impossible to split up every transparent object into individual pixels or polygons for sorting individually, so Panda sorts objects at the Geom level, according to the center of the bounding volume. This works well 95% of the time.

You run into problems with large flat polygons, though, since these tend to have parts that are far away from the center of their bounding volume. The bounding-volume sorting is especially likely to go awry when you have two or more large flats close behind the other, and you view them from slightly off-axis. (Try drawing a picture, of the two flats as seen from the top, and imagine yourself viewing them from different directions. Also imagine where the center of the bounding volumes is.)

Now, there are a number of solutions to this sort of problem. No one solution is right for every situation.

First, the easiest thing to do is to use M_dual transparency. This is a special transparency mode in which the completely invisible parts of the object aren't drawn into the Z-buffer at all, so that they don't have any chance of obscuring things behind them. This only works well if the flats are typical cutouts, where there is a big solid part (alpha == 1.0) and a big transparent part (alpha == 0.0), and not a lot of semitransparent parts (0.0 < alpha < 1.0). It is also a slightly more expensive rendering mode than the default of M_alpha, so it's not enabled by default in Panda. But egg-palettize will turn it on automatically for a particular model if it detects textures that appear to be cutouts of the appropriate nature, which is another reason to use egg-palettize if you are not using it already.

If you don't use egg-palettize (you really should, you know), you can just hand-edit the egg files to put the line:

<Scalar> alpha { dual }

within the <Texture> reference for the textures in question.

A second easy option is to use M_multisample transparency, which doesn't have any ordering issues at all, but it only looks good on very high-end cards that have special multisample bits to support full-screen antialiasing. Also, at the present it only looks good on these high-end cards in OpenGL mode (since our pandadx drivers don't support M_multisample explicitly right now). But if M_multisample is not supported by a particular hardware or panda driver, it automatically falls back to M_binary, which also doesn't have any ordering issues, but it always has jaggy edges along the cutout edge. This only works well on texture images that represent cutouts, like M_dual, above.

If you use egg-palettize, you can engage M_multisample mode by putting the keyword "ms" on the line with the texture(s). Without egg-palettize, hand-edit the egg files to put the line:

<Scalar> alpha { ms }

within the <Texture> reference for the textures in question.

A third easy option is to chop up one or both competing models into smaller pieces, each of which can be sorted independently by Panda. For instance, you can split one big polygon into a grid of little polygons, and the sorting is more likely to be accurate for each piece (because the center of the bounding volume is closer to the pixels). You can draw a picture to see how this works. In order to do this properly, you can't just make it one big mesh of small polygons, since Panda will make a mesh into a single Geom of tristrips; instead, it needs to be separate meshes, so that each one will become its own Geom. Obviously, this is slightly more expensive too, since you are introducing additional vertices and adding more objects to the sort list; so you don't want to go too crazy with the smallness of your polygons.

A fourth option is simply to disable the depth write on your transparent objects. This is most effective when you are trying to represent something that is barely visible, like glass or a soap bubble. Doing this doesn't improve the likelihood of correct sorting, but it will tend to make the artifacts of an incorrect sorting less obvious. You can achieve this by using the transparency option "blend_no_occlude" in an egg file, or by explicitly disabling the depth write on a loaded model with node_path.set_depth_write(false). You should be careful only to disable depth write on the transparent pieces, and not on the opaque parts.

A final option is to make explicit sorting requests to Panda. This is often the last resort because it is more difficult, and doesn't generalize well, but it does have the advantage of not adding additional performance penalties to your scene. It only works well when the transparent objects can be sorted reliably with respect to everything else behind them. For instance, clouds in the sky can reliably be drawn before almost everything else in the scene, except the sky itself. Similarly, a big flat that is up against an opaque wall can reliably be drawn after all of the opaque objects, but before any other transparent object, regardless of where the camera happens to be placed in the scene. See howto.control_render_order.txt for more information about explicitly controlling the rendering order.

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