Censoring image in GIMP

Question

I need to censor some images (Whole image).

I need these images for a essay about pornography in a webpage and I need the images (pornographic pictures) be censored (something like a pixelized image so the viewer should be able to understand it's about pornography but shouldn't be able to see the original image).

I'm using GIMP.

I used "Filters->Blur->Pixelize".

But my problem is, when the viewer zooms out on image, he can see the original image! (When the image gets very small -- less than 200px or 100px -- the original image is detectable just like a low resolution picture that is clearer when it's zoomed out and is smaller).

How can I overcome this? (for example by randomizing pixels when pixelized)

Answer 1

The problem is that pixelization, like blurring, effectively acts as a low-pass filter, removing fine details from the image but leaving the coarse shape unchanged. When you look at an image from a distance, all you see is the coarse shape anyway, and so the image appears unchanged.

The effect is particularly noticeable with pixelization, since, unlike plain blurring, it also introduces artificial high-frequency artifacts — the pixel boundaries — that distract the eye when seen from up close. This makes pixelization more efficient than blurring in obscuring details at short distances, but also means that a pixelized image that looks unrecognizable when looked at closely may in fact turn out to be perfectly recognizable when viewed from afar.

Unfortunately, there is no simple solution to this problem that would work in all cases. In your question, you write that:

"[the] viewer only should be able to understand its about porn and shouldn't be able to findout what is the original image."

In that case, I would suggest cropping the images so that there's not enough context for the viewer to tell what they actually represent. In fact, you could even get away with using images that are not pornographic at all — the web is full of pictures of perfectly innocuous body parts (like elbows, fingers and eyelids) that just happen to look like something else when suitably cropped.

Answer 2

Why don’t you cover the explicit parts of the photo with a black box?

Or if you are set on using filter > blur > pixelize… Select the area of the image you want censored then apply the filter to that area. If by zooming out, you can still recognize what the image is about then increase the amount of pixelization from the dialog box that pops up when you try applying the filter.

Answer 3

'Blur' filters only remove details, but keep the overall shape of what's being obfuscated intact, leading to the zoom-out issue you've described. Ilmari's answer does a good job of explaining this.

However, you can actually scramble the pixels in a selected area to make the image indistinct, which can help you around this.

Try using Filters → Noise → Spread to randomly move around the actual pixels in the area you're attempting to obfuscate.

Play around with the 'Spread' values a bit. 30-50% of the width/height of the selected area works well to remove details while maintaining the overall shape, but if you're censoring more... intimate details, you may want to use much higher spreading values.

This should distort the image enough that you can now use Filters → Blur → Pixelize to get that classic Censored image look, but now it is effective at any distance because what you're burring is nonsense instead of an actual image.

Side note: This method works for text, as well.