i = Import["http://i.stack.imgur.com/4VRfG.png"];
j = MorphologicalBinarize[i];
k = EdgeDetect[j, 7];
lines = ImageLines[k, .15];
jw = Image[ImageData[j] //. 0 -> 1];
sh = Show[jw, Graphics[{Thick, Black, Line /@ lines}]];
er = Erosion[sh, 4];
ColorNegate@Closing[ColorNegate[er], 5]