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01-13-06, 01:59 AM   #1
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Demonstration line drawing code

Since a couple of people expressed interest in how to do this, I've put together some code which shows how you can use SetTexCoord in 1.9 to draw lines using Texture objects.

The code can be found at http://www.vigilance-committee.org/wow/downloads/


It's got a silly bouncing line demo in it, and you can look at the code (Though i'm going to include that below).

Here's how it works (logically)

1. A line between two points can be thought of as a rectangle of a desired width and length, positioned (translated and rotated) to run from the first point to the second.

2. I take a square image, scale it in each direction to be the right shape, and rotate it to have the right orientation, then put it into a Texture object in the right place on the screen.

In actuality the square image has to have a transparent border of pixels, so there's a slight bit of adjustment for that, but it's the basic idea.

Put into code using a 256x256 texture (254x254 solid, with a 1 pixel transparent border) it's:
local LINEFACTOR = 256/254; -- Multiplying factor for texture coordinates
local LINEFACTOR_2 = LINEFACTOR / 2; -- Half o that

-- T        - Texture
-- C        - Canvas Frame (for anchoring)
-- sx,sy    - Coordinate of start of line
-- ex,ey    - Coordinate of end of line
-- w        - Width of line
-- relPoint - Relative point on canvas to interpret coords (Default BOTTOMLEFT)
local function DrawLine(T, C, sx, sy, ex, ey, w, relPoint)
   if (not relPoint) then relPoint = "BOTTOMLEFT"; end

   -- Determine dimensions and center point of line
   local dx,dy = ex - sx, ey - sy;
   local cx,cy = (sx + ex) / 2, (sy + ey) / 2;

   -- Normalize direction if necessary
   if (dx < 0) then
      dx,dy = -dx,-dy;

   -- Calculate actual length of line
   local l = sqrt((dx * dx) + (dy * dy));

   -- Quick escape if it's zero length
   if (l == 0) then
      T:SetPoint("BOTTOMLEFT", C, relPoint, cx,cy);
      T:SetPoint("TOPRIGHT",   C, relPoint, cx,cy);

   -- Sin and Cosine of rotation, and combination (for later)
   local s,c = -dy / l, dx / l;
   local sc = s * c;

   -- Calculate bounding box size and texture coordinates
   local Bwid, Bhgt, BLx, BLy, TLx, TLy, TRx, TRy, BRx, BRy;
   if (dy >= 0) then
      Bwid = ((l * c) - (w * s)) * LINEFACTOR_2;
      Bhgt = ((w * c) - (l * s)) * LINEFACTOR_2;
      BLx, BLy, BRy = (w / l) * sc, s * s, (l / w) * sc;
      BRx, TLx, TLy, TRx = 1 - BLy, BLy, 1 - BRy, 1 - BLx; 
      TRy = BRx;
      Bwid = ((l * c) + (w * s)) * LINEFACTOR_2;
      Bhgt = ((w * c) + (l * s)) * LINEFACTOR_2;
      BLx, BLy, BRx = s * s, -(l / w) * sc, 1 + (w / l) * sc;
      BRy, TLx, TLy, TRy = BLx, 1 - BRx, 1 - BLx, 1 - BLy;
      TRx = TLy;

   -- Set texture coordinates and anchors
   T:SetTexCoord(TLx, TLy, BLx, BLy, TRx, TRy, BRx, BRy);
   T:SetPoint("BOTTOMLEFT", C, relPoint, cx - Bwid, cy - Bhgt);
   T:SetPoint("TOPRIGHT",   C, relPoint, cx + Bwid, cy + Bhgt);
There's a notes.txt file inside the zip above with the derivation of those cryptic values 8-)

Hopefully someone out there finds this useful.
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