SDL, sloped pixel lines

[klausvdl]

Also looking at you code it seems as if you are using a for loop to determine the width of line rectangles. Isn't that very slow?
Wouldn't it be easier to calculate the width of horizontal lines (line segments) that will represent your diagonal line by simply dividing the dx with dy?

You're right ...

I adapted a line algorithm intended for drawing single pixels. That code has to iterate over every pixel of the line. But that's unnecessary when drawing horizontal or vertical line segments instead of pixels.
So I followed your hint and rewrote the code:

procedure draw_line(renderer: psdl_renderer; x1, y1, x2, y2: longint);
var
  dx, dy, rx, ry: longint;
  error, c: longint;
  step, rest: longint;
begin

  { if the whole line is horizontal or vertical than use SDL_RenderDrawLine and exit
    if not, we know that neither dx nor dy can be zero (division by zero not possible) }
  if (y1=y2) or (x1=x2) then begin
    sdl_renderdrawline(renderer, x1, y1, x2, y2);
    exit;
  end;

  dy := y2-y1;
  if dy>=0 then ry := 1
  else begin
    dy := -dy;
    ry := -1
  end;
  inc(dy);

  dx := x2-x1;
  if dx>=0 then rx := 1
  else begin
    dx := -dx;
    rx := -1
  end;
  inc(dx);

  { slope is moderate (dy/dx <= 1) }
  if dx>=dy then begin

    step := dx div dy;
    rest := dx-(dy*step);
    step := step*rx;

    x2 := x1;

    error := dy shr 1;
    for c := 1 to dy do begin

      dec(error, rest);
      if error<0 then begin
        inc(error, dy);
        inc(x2, rx);
      end;

      inc(x2, step);
      sdl_renderdrawline(renderer, x1, y1, x2-rx, y1);
      x1 := x2;

      inc(y1, ry);
    end;

  end
  { slope is steep (dy/dx > 1) }
  else begin

    step := dy div dx;
    rest := dy-(dx*step);
    step := step*ry;

    y2 := y1;

    error := dx shr 1;
    for c := 1 to dx do begin

      dec(error, rest);
      if error<0 then begin
        inc(error, dx);
        inc(y2, ry);
      end;

      inc(y2, step);
      sdl_renderdrawline(renderer, x1, y1, x1, y2-ry);
      y1 := y2;

      inc(x1, rx);
    end;

  end;
end;

Regarding your other question: I have to know the signs of the deltas (dx, dy) and store them in the rates (rx, ry) for later use. After that I make the deltas positive.

Thanks for following my trial-and-error technique.