Blurring, as a tool for obscuring personal information in images, is both
ill-advised and exceedingly common.
Screenshot blurred using GIMP
2.8's Gaussian blur tool, 8px radius.
One especially bad choice is box blur
on monochrome text. Consider a 7×7px box blur:
- Each output pixel value is the average of the 49 input pixel values in
the surrounding 7×7px box.
- The number of white input pixels in the 7×7px box around an output
pixel with value v ∈ [0.0, 1.0] is round(49v).
- Therefore for any 7×7px box with 48 known input pixels, the 49th
input pixel can be computed directly.
- Therefore if the first six rows and first six columns are known, then
the entire input image can be recovered by induction.
Monochrome text, 7×7px box blur.
A more popular choice of blur is pixelization, in which the image is divided
into a grid and each grid square is filled with its average value. Unlike
box blur, pixelization is not invertible for monochrome images. However,
it is still easy to recover text because a only a few bits of information
are required to deduce each character when the character set is known.
Courier New 11px.