parallel hough algorithm using omnithread lib

I want to speed up image processing using the hough circle detection.

   // For all rows in image:
   for y:=0 to AnalysisBitmap.Height-1 do
   begin
   // For all pixel in one row :
   for x:=0 to AnalysisBitmap.Width-1 do
   begin
      // Is there a point  ?
      if IsPixel(x,y, AnalysisBitmap, 128 ) then
      begin
           for theta:=0 to max_theta do
           begin
                TestPoint.x := round ( x -  r  *  cos(theta*PI/max_theta) );
                TestPoint.y := round ( y -  r  *  sin(theta*PI/max_theta));

               if ((testPoint.x < ImageWidth) and  (testPoint.x > 0 )  and
                  (testPoint.y < ImageHeight ) and  (testPoint.y > 0 ) )   then Inc(aHoughResult[TestPoint.x,TestPoint.y]);
           end;
      end;
   end;
  end;

As the VCL Bitmap is not thread safe I guess I can only do parallel processing of the inner Theta Loop ? What is the best Approach to Speed up this code .

Yes, it is enough to parallelize the inner cycle only. Don't forget to organize right sharing of aHoughResult, for example - with critical section.

In the newest Delphi versions you can use both OTL and inbuilt System.Threading.TParallel possibilites.

The most important speedup (I think) - fill the table with round(r*cos(theta*PI/max_theta)) values and use it inside the cycles.