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I have got a four 2d vertices A B C D of rotated rectangle, I need to rasterize/draw it (efficiently) in pixelbufer with setpixel(x,y,color)

how to do it?

i was trying with some code like

    // convertilg a b c d do up down left right, 
    // calculating some dx_left dx_right on y--
    // etc (frustrating on special cases when there are 2 up_y vertices in same line etc)


    for(;;)
    {

     drawhorizontalline(y, xstart, xend, color);

     if(y==downy) break;

     y--;
     xstart+=dxstart;
     xend+=dxend;

     if(y==lefty)  dxstart = dxright;
     if(y==righty) dxend = dxleft;

     }

but it is most frustrating (terribly bug prone and most frustrating) i am really tired of debuging this all day yesterday and i need to find maybe some working code rather than to try to debug this

grunge fightr
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  • You'd normally start by implementing a line drawing algorithm (e.g., Bresenham's), and then invoke it four times, one for each line of the rectangle. Googling for "Bresenham's algorithm" (or something similar) should turn up a *lot* of results. – Jerry Coffin Apr 08 '12 at 07:40
  • I need filled rectangle - and rasterizning by horizontal lines would be faster - like in my approach - but im tired of debug this – grunge fightr Apr 08 '12 at 07:54
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    [This question/answer](http://stackoverflow.com/q/7870533/968261) will help you get 95-99% of the work. – Alexey Frunze Apr 08 '12 at 08:56
  • Filling a rectangle with lots of calls to `setpixel` is usually not very efficient. – n. m. could be an AI Apr 08 '12 at 14:51
  • @grunge fightr well you seen inactive at this question for a while do you still need an better answer or you have done it already. If you need specific code comment me with what params you need (color,texture,...) – Spektre Feb 02 '14 at 10:36

1 Answers1

11

To fill your rectangle handle it as closed convex polygon (almost the same as triangle filling)

  1. order your points to match winding rule

    so there are lines AB BC CD DA or reverse

  2. create left and right buffer

    address is y-coordinate, its an array of x-positions and if needed also array of color,texture coordinates,.... for starters:

    int buf_x0[ys],buf_x1[ys];

    where ys is screen y-resolution

  3. implement any draw line algorithm

    but instead of draw to screen just store x coordinate of pixel to buffer.

    • instead of: setpixel(x,y,color); do: buf_x?[y]=x;.

    Which buffer is the destination depends on the line Y direction

    • if dy<0 then fill buff_x0
    • if dy>0 then fill buff_x1
    • if dy==0 then buf_x0[y]=min(x) and buf_x1[y]=max(x)

    Beware you have to sort the line endpoints by x coordinate before rasterizing to avoid seams/holes in meshes caused by different pixels produced for reversed endpoints lines.

  4. apply this line algorithm to all border lines of polygon (AB,BC,CD,DA)

    after this the buffers contains start and end x-positions of your horizontal lines

  5. fill the rectangle on screen

    for (y=min(Ay,By,Cy,Dy);y<=max(Ay,By,Cy,Dy);y++)
 draw_horizontal_line(y,buf_x0[y],buf_x1[y],color);

Image for clarity (taken from my lectures on low level computer graphics)

border buffer creation

image description:

  • vertical rectangles represents the border buffers buf_x0[],buf_x1[]
  • clockwise winding rule ensures the destination buffer. If its coded properly than buf_x0[y] <= buf_x1[y] so draw of horizontal line colapses to single for loop

Also se these:

Spektre
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