Rendering squares in 3D

Question

Taking into account the position of the square (in this case, a wall), the position and rotation of the "camera" and other factors, I know how to calculate where the four points of the square should be when rendered to a 2D canvas.

What I don't know, however, is how to actually render the square. If I have an image, how do I draw it so that its four corners are positioned on the four points I've calculated?

More information: What I'm actually trying to do is render a view inside a maze. I have previously done this using raycasting, similar to Doom and Wolfenstein 3D. However, this only worked well for smaller views (320x240 being the one I settled on). I'd like to render this in fullscreen. Any advice for rendering such a scene (square walls, sprite-based objects, etc.) would be appreciated.

possible duplicate of [Image manipulation and texture mapping using HTML5 Canvas?](http://stackoverflow.com/questions/4774172/image-manipulation-and-texture-mapping-using-html5-canvas) — 6502, Feb 24 '13 at 17:34
Are you trying to render a solid square, or render an image that is rectangular? — Mark Hildreth, Feb 24 '13 at 17:42

C.. · Accepted Answer · 2013-06-12T14:30:43.870

Not entirely sure, but something along these lines is what I think you're asking for:

//Credit to Andrea "6502" Griffini.
//Pass context, image and points of triangle to draw
function textureMap(ctx, texture, pts) {
  var x0 = pts[0].x, x1 = pts[1].x, x2 = pts[2].x;
  var y0 = pts[0].y, y1 = pts[1].y, y2 = pts[2].y;
  var u0 = pts[0].u, u1 = pts[1].u, u2 = pts[2].u;
  var v0 = pts[0].v, v1 = pts[1].v, v2 = pts[2].v;

  ctx.save(); ctx.beginPath();
  ctx.moveTo(x0, y0);
  ctx.lineTo(x1, y1);
  ctx.lineTo(x2, y2);
  ctx.closePath(); ctx.clip();

  var delta = u0*v1 + v0*u2 + u1*v2 - v1*u2 - v0*u1 - u0*v2;
  var delta_a = x0*v1 + v0*x2 + x1*v2 - v1*x2 - v0*x1 - x0*v2;
  var delta_b = u0*x1 + x0*u2 + u1*x2 - x1*u2 - x0*u1 - u0*x2;
  var delta_c = u0*v1*x2 + v0*x1*u2 + x0*u1*v2 - x0*v1*u2 - v0*u1*x2 - u0*x1*v2;
  var delta_d = y0*v1 + v0*y2 + y1*v2 - v1*y2 - v0*y1 - y0*v2;
  var delta_e = u0*y1 + y0*u2 + u1*y2 - y1*u2 - y0*u1 - u0*y2;
  var delta_f = u0*v1*y2 + v0*y1*u2 + y0*u1*v2 - y0*v1*u2 - v0*u1*y2 - u0*y1*v2;

  ctx.transform(delta_a/delta, delta_d/delta, delta_b/delta, delta_e/delta, delta_c/delta, delta_f/delta);
  //Edit to allow for repeated image
  var x_max = Math.max(u0,u1,u2); var x_min = Math.min(u0,u1,u2);
  var y_max = Math.max(v0,v1,v2); var y_min = Math.min(v0,v1,v2);
  var x_iter = x_max-x_min; var y_iter = y_max-y_min;

  for (var x=0; x<x_iter; x+=texture.width) {
    for (var y=0; y<y_iter; y+=texture.height) {
      ctx.drawImage(texture, x, y);
    }
  }
  ctx.restore();
};

With the effect of rendering like the crate on the right in: http://www.placidcow.hostoi.com/pages/ExGl%20Crate.htm

Hope helps!

Rendering squares in 3D

1 Answers1