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One of the great uses for the Difference mode is that it allows you to compare seemingly identical images and discover their differences. And here's how that works. We are looking at a photo from Fotolia image library, I absolutely love it because it's a great demonstration of Blend Modes, we have got this photograph in the background, then this falling type in the foreground set to the Screen mode I would imagine, in order to create this bright interaction. The name of the file is Access codes. tiff so I save the image to the tiff file format, I use LZW compression, which is entirely lossless.
So it doesn't change a single pixel inside the image. Meanwhile, next-door, it looks like the same image; we will see that it's not in a moment. It's called Heavily compressed.jpg, and what I did was, I save that same file to the JPEG format, and I applied the lowest quality setting possible, which is 0. And as a result, if you go and zoom in on this image, you can see that we have all kinds of compression artifacts, they show up as these 8x8 pixel squares, and basically all of the pixels are rewritten, based on the color of the top left pixel and that's how JPEG works.
Now let's say we want to get a sense of what exactly has change between these two images. Well, I am going to right click inside this image and choose Duplicate Layer and then send it over to Access codes.tiff and change its name to let's say JPEG version and click OK. And now we'll switch back to the image at hand. To compare the two, I will go ahead and change the Blend Mode from Normal to Difference. Now first it's going to look like the image is gone completely black, so there must not be any differences between the pixels, because identical pixels set to the Difference mode cancel each other out.
However, that's not actually the case. So what I am going to do is press the Escape key to deactivate the Blend mode there. And I'm going to create a Merged version of these two layers by pressing Ctrl+Shift+Alt+E or Command+ Shift+Option+E on the Mac. We will go ahead and create a merged composite on a new layer. I will go ahead and call this new layer Merged as well. Now what you need to do is go up to the Image menu, choose the Adjustments command and choose Levels or you can press Ctrl+L, Command+L on the Mac and look at that histogram, there is some action going on in the very dark regions of this layer.
To make it evident, go ahead and grab that white slider triangle and move it very far over to the left hand side. And I am going to ultimately settle on a white point value of 20. So that we have a fair amount of distinction going on here and then I will click OK. Any pixel at this point that does not appear black has been rewritten, which means that applying a lot of JPEG compression in our case, has rewritten just about every single pixel in the image.
So any time you want to be able to compare two seemingly identical images and figure out if they are truly the same thing, then you can do this exact same tests we did here, using the Difference Blend Mode, going ahead in merging the two layers and then using the Levels command to exaggerate the differences.
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