Assignment 14: Transparency in Meshes

This week’s game: simple one-click download below, unzip, play, enjoy!

NancyNewren_ExampleGame 14

What’s new
I added two transparent objects!

Main object: Use arrow keys to move main object left/right/up/down
Left/Right (A/D), Up/down (W/S), Zoom in/out (Q/E)

(When moving the camera things in the world will appear to move opposite. Thus when you move camera left, the world will appear to move right.)

 About the Project

This time around I added the ability to become transparent to my meshes! It was fun.

First thing was to create a shader and an effect to allow for transparency. Next I created two objects drawn with them, but drew the meshes in whatever order they were submitted to the render thread. This works fine for opaque objects, However for transparent objects when a transparent object is drawn in before an object behind it is drawn, the object behind it isn’t drawn properly (as expected that is) to the screen. I.E. you can’t see the object behind a transparent object if that object isn’t drawn before it.

This means that all transparent objects have to be drawn back to front. The order of the opaque objects doesn’t matter though (with depth buffering on at least), as long as they are drawn before all transparent objects.

So what I did to order the draw calls is: after submission to the render thread is over and and before the swap, I ordered the draw calls. How? First thing: I added the index of all opaque objects (I just asked the mesh’s effect if it was opaque, nicely of course) to a vector in the order they were submitted, and then put the transparent meshes in a separate vector with their index and their camera space z value. This is the key to quickly sorting these meshes: in camera space I immediately know which meshes are in the back and in the front based solely on their z values since camera space is when the camera is at the origin and is facing forward. Then a simple std::sort with a lambda function that orders the z’s based on z forward being negative z (i.e. larger values of z aren’t closer to the camera, they are the farthest away), quickly sorts those values. Once sorted I added the index values of the sorted transparent meshes to the end of the previously mentioned vector (the one with the opaque meshes all in front to be drawn first.) Then in the rendering of the meshes I draw them in the order of the sorted draw calls.

I chose the render thread because with my game it doesn’t really matter which thread does the sorting because it’s such a small game. However, in a real game I would profile both threads and then choose the thread that takes the least amount of time.


I asked my brother how to use the std::sort with my vector<struct> as I hadn’t done a templated sort before . He set me on the path with the google search I needed and I whipped out a lambda compare for my sort in no time.  Thanks bro!


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