A very large output was generated here is a sample of it...

I am trying to do edge detection on an image and instead of getting the processed image I get a bunch of numbers with this remark ‘A very large output was generated here is a sample of it’.

I don’t understand what the issue is. Please bear with me. I am new to Mathematica but proficient in Matlab.

This is the code I run:

im=Table[If[x^2+y^2<7000,100,0],{x,-128,127},{y,-128,127}];
p1=(RasterPlot[#1]&)/@{im,gDc[im,1,0,1]};
GraphicsRow[p1,ImageSize->350]

What is RasterPlot ? What is gDc ?

These are functions from a specific library:http://www.mathvisiontools.net/

It is somewhat difficult to debug your example without installing the mathvisiontools package. However, since you mention edge detection, you may wan to explore the many image processing functions that are part of Mathematica. For example:

http://reference.wolfram.com/mathematica/ref/EdgeDetect.html

And part of your example might go like this:

image = Image[
   Table[If[x^2 + y^2 < 7000, 100, 0], {x, -128, 127}, {y, -128,  127}]];

EdgeDetect[image]

Which yields