Find Black Squares/Rectangles in Image with C# -

i want find small black squares/rectangles in scanned sheet to:

1. deskewing image if needed.
2. remove white page margin.

input image: sample image 1 http://us.cdn.persiangig.com/preview/ii2jf6/2.jpg

output image: sample image 2 http://us.cdn.persiangig.com/preview/9ntpnc/1.jpg

my code find square is:

bitmap pic =(bitmap) picturebox1.image;  // create filter aforge.imaging.filters.median medianfilter = new aforge.imaging.filters.median(); // apply filter medianfilter.applyinplace(pic);  bitmap grayimage;  if (pic.pixelformat != pixelformat.format16bppgrayscale && pic.pixelformat != pixelformat.format8bppindexed) {     aforge.imaging.filters.grayscale grayscalefilter = new aforge.imaging.filters.grayscale(0.2125, 0.7154, 0.0721);     grayimage = grayscalefilter.apply((bitmap)picturebox1.image); } else {     grayimage = pic; }  // black & white: threshold(ref grayimage, convert.toint32(textbox1.text), convert.toint32(textbox1.text));  // invert filter invert invertfilter = new invert(); // apply filter invertfilter.applyinplace(grayimage);  // edge detector  filter differenceedgedetector edgedetectorfilter = new differenceedgedetector(); // apply filter edgedetectorfilter.applyinplace(grayimage);  // create filter dilatation dilatationfilter = new dilatation(); // apply filter dilatationfilter.applyinplace(grayimage); 

finding object(square/rectangle) in image:

        // lock image         bitmapdata bitmapdata = grayimage.lockbits(new rectangle(0, 0, grayimage.width, grayimage.height),         imagelockmode.readwrite, grayimage.pixelformat);          // step 2 - locating objects         blobcounter blobcounter = new blobcounter();          blobcounter.filterblobs = true;         blobcounter.minheight = 10;          //*-*-*-*-         blobcounter.minwidth = 10;         blobcounter.maxheight = 50;         blobcounter.maxwidth = 50;          blobcounter.processimage(bitmapdata);         blob[] blobs = blobcounter.getobjectsinformation();         grayimage.unlockbits(bitmapdata);          // step 3 - check objects' type , highlight         simpleshapechecker shapechecker = new simpleshapechecker();          graphics g = graphics.fromimage(pic);         pen redpen = new pen(color.red, 2);       // quadrilateral         pen bluepen = new pen(color.blue, 2);     // triangle          (int = 0, n = blobs.length; < n; i++)         {             list<intpoint> edgepoints = blobcounter.getblobsedgepoints(blobs[i]);              list<intpoint> corners;              // triangle or quadrilateral             if (shapechecker.isquadrilateral(edgepoints, out corners))             {                 // sub-type                 polygonsubtype subtype = shapechecker.checkpolygonsubtype(corners);                  pen pen;                  if (subtype == polygonsubtype.square)                 {                     pen = (corners.count == 4) ? bluepen : redpen;                     g.drawpolygon(pen, topointsarray(corners));                 }             }         }         redpen.dispose();         bluepen.dispose();         g.dispose();         picturebox1.image = pic; 

problem low accuracy on detecting squares , rectangles!!!

how can solve problem?

if can use opencv quite easy. use hough transform , find peaks in output. correspond straight lines in input.

if can't use opencv, you'll need implement yourself. here's started.

https://en.wikipedia.org/?title=hough_transform

edit

as anders gustafsson points out in comment below, hough transform available in aforge .net implementing isn't necessity.

http://www.aforgenet.com/framework/features/hough_transformation.html