PNG's with Translucency, and Optimization

Question

Hi everyone, I've been working on optimization for my levels but I'm a bit confused about translucency. One common tactic I've noticed in games is to reduce what could be a complex model into a single surface.

An example from Dishonored:

alt text

I've seen countless examples of this in games, but what I don't understand is why the textures never appear to have anti aliasing, leaving very rough edges between the solid and the transparent like in the example above.

Here's a grass texture I found:

alt text

This one has a translucent edge and looks much better. Is there a reason that the pixels of the PNG should only be at 100% opaque or 0% opaque and that anything between would slow things down? I have tested this myself in Unity and I see no performance difference between either method. But if this is the case, than why do so many games do this? Is there a reason I should be avoiding translucency when using PNGs like this? I would really appreciate any suggestions.

Answer 1

Answer by supernat · May 26, 2014 at 05:32 AM

In shaders (this goes back to the original fixed function pipeline design), you have two options with regard to alpha: 1) reject a pixel with an alpha value less than some threshold (Alpha Testing) (which is what you see) or 2) blend the alpha with the background. By design, the 1st is much faster (it used to be anyway), because you don't have to perform any blending. The pixel in the non-opaque areas is just immediately tossed out and the color buffer remains with the last color. When you alternatively use the 2nd approach, you have to blend all of the texture, so even places with zero opacity will take a hit, and the actual blending itself is a bit heavy. They don't specifically call this out as the reason on this linked page, but I think they are doing a two pass render to optimize the speed: https://docs.unity3d.com/Documentation/Components/SL-AlphaTest.html

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PNG's with Translucency, and Optimization

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