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Optimization and AI

July 2nd, 2012 by Christian Knudsen

I finally got the last big optimization working last week – well halfway. Explanation follows.

The way I do the x-ray effect in Hostile Takeover is to draw the area around the player character without drawing walls in front of him nor any other objects that you should be able to see through. This is drawn to the back buffer and never actually shown on screen. I then read this from the back buffer into a 128×128 pixel image and change the alpha value of the pixels so that the further they are from the image center, the more transparent they are. I then draw the screen as usual and finally draw this 128×128 image back on top of this.

The problem with this is the part where I read the pixels from the back buffer into the 128×128 texture. The whole rendering pipeline is optimized for writing/uploading information to the GPU and framebuffer, not reading/downloading. So on older systems, this can create a bit of a bottleneck. The solution is to use a framebuffer object (FBO) instead. FBOs were an extension to OpenGL that I believe now has become a core feature. Instead of writing to the back buffer and then reading back the information in order to apply the gradual transparency and writing the whole thing to a texture, I can use FBOs so that a texture can be set up to work like a framebuffer, meaning that I can draw everything directly to the texture – cutting out the bottleneck of reading pixel information from the back buffer.

It all works fine and dandy and quite a bit faster – at least when running in a window. I’m having an issue with the 16-bit depth of the game messing up the FBO in fullscreen. So for now, it’ll just fall back to the slower method when running in fullscreen, and I probably won’t be spending more time on this for the pre-alpha release. But if you’re running the game on a computer that isn’t more than 5 years old, you probably won’t have any issues with this anyway. And if you do, you can just run in window mode for now.

So that’s off my to-do list for now. Which means that I’m now finally fully into AI. The first thing I did was extending the pathfinding code to work across multiple layers, meaning that NPCs now know how to use elevators. Following that, I’m focusing on patrolling guards and how they’ll react to spotting you.

Click to enlarge

Click to enlarge

There are three types of areas, color coded green, yellow and red on the map. In a green area, your stealth level can’t decrease. In a yellow area, it’ll decrease if you get spotted doing anything you shouldn’t be doing. And in a red area, you’ll lose stealth as soon as you’re spotted – and if you don’t leave fast enough (of if the guards are already suspicious of you), you’ll get attacked. All that is working, except for the attacking part. So I’m currently teaching my little NPC dudes how to shoot guns. They’re slowly getting the hang of it.


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