Prototyping new game ideas is a lot of fun. Thanks to some serious enhancements to my game framework it’s also pretty quick.

I enjoy day-dreaming new ideas and recently I was watching something on TV that put me in mind of the film Star Wars. It was a fast paced scene in a TV show with cars, helicopters and trucks all viewed from above. I was struck by how effective the shot was in portraying speed with the buildings that lined the streets.

The Star Wars link came with the closing scenes in the first film as the fighters raced down the trench. It occured to me that this scene viewed from above might work quite well for a (yet another !) shooting game. I quite liked the claustrophobia of it, amongst other things.

So I had a bash at putting together a short test.
The theory was quite simple. I would use a simple fillRect() to block out the 5 key areas of the trench. The lighter shades at the edge would of course be the top of the trench (the surface if you will of the Death Star) and the darker shades would represent the inner walls and the trench’s floor.
You can see the rectangular divisions highlighted in the image below.

All I needed to do was give the impression of movement.
For this I am using lines and paths.

I created a trenchLine() class and initiated it with 2 y values. Nothing more is needed at the moment.
The first y co-ordinate is for the first 2 points and the last 2 points (see the image below).
the second co-ordinate is for the middle points which will be moving that bit slower.

As the game begins I initiate around 4 trench lines to be drawn over the grey background at equidistant intervals down the screen. (In the illustration below the 4th line is just off the top of the canvas so not yet visible)
The key here is in how the trench line breaks up to represent the pseudo-3D nature of the trench.

click to enlarge

So I originate the line at x = 0.
The line itself is broken in to 5 shorter lines.
On a 320 wide play area I draw the line’s points at

1. x=0, // left hand side
2. x=32, // edge of trench – about to drop to floor
3. x=48, // left hand side of the 192px wide floor
4. x=240, // right hand side of trench floor
5. x=288 // edge of trench on right hand side

Each point moves at a set speed down the screen.
I set the trench speed to be, say, 4 pixels per frame.
This affects points 1,2, 5 and 6.

But the trench floor needs to move slower and for these points (3 and 4) I move them at 3 pixels per frame.

I have a simple line drawing routine:

[code lang=”javascript”]

function drawLine(x1,y1,x2,y2,c)
g.ctx.moveTo(x1, y1);
g.ctx.lineTo(x2, y2);
g.ctx.strokeStyle = c;
catch (e)
write("drawline: " + e.message);


Quite straight forward in that it accepts the start and end points of the line and the colour that you wish to draw it.
I then pass the following 5 draw instructions to it to create my trench line:

[code lang=”javascript”]



The overall effect here is that the outer lines effectively overtake the inner lines and you get the impression of a moving trench from above – as per the screen shot.

I created some placeholder sprites and threw them around to see how effective playing the game in a narrow 192px corridor might be. So far so good.

Watch this space :)

4 Responses

    • Thanks, John. I’m all for these simple and inexpensive tricks :) I figure that if I can create an interesting backdrop cheaply I get to spend a bit of horsepower on the finer details of the game. It’s coming together nicely just now.

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