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I really want to know how the bullets in Radiangames Inferno are done. The bullets seem like they are just billboard particles but I am curious about how their tails are implemented. They can curve so this means they are not just a billboard. Also, they appear continuous which implies that the tails are not made of a bunch of smaller particles (I think).

Can anyone shead some light on this for me?

screen shot

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I'll bet the developer would answer you directly if you emailed him: radiangames.com/?page_id=2 –  Steve H Feb 21 '11 at 14:37
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2 Answers

up vote 3 down vote accepted

It's easy enough if you're doing 2D with a 3D API, and can draw textured polygons.

Store a history of previous positions, use these to build a triangle strip along the bullet's path. Then texture it, and draw with additive blending

(However, if the bullets make any sharp turns, or you need wide trails, it may get more interesting - you'll have problems creating clean geometry without unwanted overlaps)

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One of the many awesome features of XNA is that you can apply texture shaders to 2D sprites, enabling you to achieve cool effects like the one in the image. XNA is based on Direct3D, so you have access to any shaders written in HLSL (High-level Shader Language). If you are new to shaders and their applications, check out this awesome MSDN article that explains how to actually implement them in an XNA draw call.

This site contains an awesome example of a Gaussian blur post-processing effect that can probably be tweaked around a bit to achieve an effect like the one pictured. Note that this particular shader is written in GLSL, however, so while the concepts remain constant, the syntax will differ ever slightly from HLSL.

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