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# 2d to Pseudo 3d in Android

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2 replies to this topic

Posted 08 May 2012 - 08:31 PM

I need some help redrawing a 2d view into pseudo 3d. I've made a simple game app that draws horizontal lines across the screen to create rectangles in alternating colors. When the top of the screen is touched the rectangles move vertically down the screen, creating the illusion of movement. As it's being drawn currently, it looks like you are looking down on the moving rectangles from above.

What I'd like to do is take those rectangles and transform them to look like they are 3d, from a first person style view. I've seen a lot of examples of taking a known 3d point and converting it to a 2d point on the screen, but I'm not sure how to apply that to what I'm doing. Since the lines are being drawn in 2d, there is no 3d point for me to transform.

The code for drawing a line is pretty simple:
canvas.drawLine(Xstart, Ystart, Xend, Yend, paint);

So given the start and end point of each line in 2d, how can I redraw them to look like you are looking out over a horizon of rectangles rather than
looking straight down at them from above?

### #2danbrown  Members

Posted 09 May 2012 - 12:59 PM

The easiest way of getting perspective (and how we did 3d pre-hardware acceleration) is to divide your screen coordinates by the required depth. The coord 0,0 will need to be in the screen center for it to look right.

X and Y remain the usual screen coord system, Z is the depth. A depth of 1 will give you the same pixel-mapping as before. Greater than 1, X and Y will shrink to look like it is getting further 'into' the screen. Less than 1, X and Y get bigger as if they are coming 'out' of the screen.

It sounds like you want a guitar-hero fret look to your rectangles? You will need to define them in X (constant for left and right edges), a constant Y and Z increasing 'into' the screen.

Note that this is about the simplest 3d system you can get. You can hack in field-of-view, basic parallel viewpoint movement and some rotation, but for anything more complex/generic you are better off looking into matrix stacks. Or just make the switch to OpenGL ES as that is what you will be emulating anyway.

Dan

Posted 10 May 2012 - 12:12 AM

The guitar hero comparison is a perfect example of what I want to accomplish visually. I wish I had thought of that to get my point across better. There are a couple of things I'm still not quite clear on though.

So I draw my line rectangles with this code:

for (int i = 0; i < 20; i++){

Ystart = this.gridHeight;
Yend = this.gridHeight / 2;
Xstart = this.gridWidth / 2 + i;
Xend = this.gridWidth / 2 + i;
canvas.drawLine(Xstart, Ystart, Xend, Yend, paint);
}

This gives me the 'over the top' style view I want to transform. How do i actually determine what Z, the depth, is going to be? Is it the height of the rectangle I've created with lines? For some reason grasping this is turning me stupid, probably because actual math is involved..

As a side note the line rectangle drawn above is being drawn vertically so I can view any transform more clearly.

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