alpha triangle

procedure Alpha_Triangle(p1,p2,p3: tpoint;r,g,b,ratio: byte; bit1: tbitmap);
{
p1,p2,p3 = the 3 vertices or points at the corners of the triangle
r,g,b = the red, green, and blue values being averaged with the colours behind
ratio = a value from 0 to 255 that determines how opaque the fill of the triangle will be
-> 0 for transparent, 255 for completely opaque, however,
in either of these extremes, it doesn't make sence to use this procedure
bit1 = the bitmap having the triangle drawn on
}
// this function draws a filled triangle
// by dividing it in half and stepping through
// each line
var
x,y: integer;
height: integer; // looping variables
dx_right,dx_left: real;
// the dx/dy ratio of the right and left edges of the line
xs, xe: real; // // the starting and ending points of the edges
temp_x, temp_y: integer;
right_x,left_x: integer;
pb: pbytearray;
w: integer; // (width of bitmap) * (# of bytes per pixel)
x2: integer; // x*(# of bytes per pixel)
miny,maxy,minx,maxx: integer;
nratio: byte;
begin
if (p1.x<0)and(p2.x<0)and(p3.x<0) then
exit;
if (p1.y<0)and(p2.y<0)and(p3.y<0) then
exit;
maxx:=bit1.width-1;
if (p1.x>maxx)and(p2.x>maxx)and(p3.x>maxx) then
exit;
maxy:=bit1.height-1;
if (p1.y>maxy)and(p2.y>maxy)and(p3.y>maxy) then
exit;
// exit if none of the triangle will be shown anyway
nratio:=255-ratio;
bit1.PixelFormat:=pf24bit;
w:=bit1.width*3;
// make sure points are in order
if (p2.y < p1.y) then
begin
temp_y := p1.y;
temp_x := p1.x;
p1.y := p2.y;
p1.x := p2.x;
p2.y := temp_y;
p2.x := temp_x;
end;
if (p3.y < p1.y) then
begin
temp_y := p1.y;
temp_x := p1.x;
p1.y := p3.y;
p1.x := p3.x;
p3.y := temp_y;
p3.x := temp_x;
end;
if (p3.y < p2.y) then
begin
temp_y := p3.y;
temp_x := p3.x;
p3.y := p2.y;
p3.x := p2.x;
p2.y := temp_y;
p2.x := temp_x;
end;
left_x := p2.x;
if p3.y=p1.y then
right_x := 10000
else
right_x := round(p1.x + (p2.y-p1.y)*(p3.x-p1.x)/(p3.y-p1.y));
if (right_x < left_x)then // messes up if right is on left
begin
temp_x := right_x;
right_x := left_x;
left_x := temp_x;
end;
/////!!DRAW TOP!!///////
//draw the triangle top
if (p1.y <> p2.y) then
begin
// compute height of subtriangle
height := p2.y - p1.y;
// set starting points
xs := p1.x;
xe := p1.x;
// compute edge ratios
dx_left := (left_x - p1.x) / height;
dx_right := (right_x - p1.x) / height;
///DRAW IT ALREADY!!!!
miny:=max(0,p1.y);
maxy:=min(bit1.height-1,p2.y);
for y := miny to maxy do
begin
pb:=bit1.ScanLine[y];
minx:=max(0,trunc(xs));
maxx:=min(bit1.width-1,trunc(xe));
for x := minx to maxx do
begin
x2:=x*3;
pb[x2]:=(pb[x2]*nratio+b*ratio) shr 8;
pb[x2+1]:=(pb[x2+1]*nratio+g*ratio) shr 8;
pb[x2+2]:=(pb[x2+2]*nratio+r*ratio) shr 8;
end;
//adjust starting and ending point
xs :=xs+ dx_left;
xe :=xe+ dx_right;
end;
end;
if (p2.y <> p3.y) then
begin
//now recompute slope of shorter edge to finish triangle bottom
// recompute slopes
height := p3.y - p2.y;
dx_right := (p3.x - right_x) / (height);
dx_left := (p3.x - left_x) / (height);
xs := left_x;
xe := right_x;
if p3.y>= bit1.height then
p3.y:=bit1.height-1; // prevent an access violation error
// draw the rest of the triangle
miny:=max(0,p2.y+1);
maxy:=min(bit1.height-1,p3.y);
for y := miny to maxy do
begin
pb:=bit1.ScanLine[y];
minx:=max(0,trunc(xs));
maxx:=min(bit1.width-1,trunc(xe));
for x := minx to maxx do
begin
x2:=x*3;
pb[x2]:=(pb[x2]*nratio+b*ratio) shr 8;
pb[x2+1]:=(pb[x2+1]*nratio+g*ratio) shr 8;
pb[x2+2]:=(pb[x2+2]*nratio+r*ratio) shr 8;
end;
//adjust starting and ending point
xs :=xs+ dx_left;
xe :=xe+ dx_right;
end; //end for
end;
end;

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