Hunting Down JPEG Artifacts with Difference Blending!
The eternal struggle anyone who shares digital images faces is how to reduce the file size while keeping important details intact in your images.
Saving your photos as JPEGs is an important step before sharing images on-line, or via email etc. But along with reducing the file size, saving your photo as a JPEG can rob that image of important details leaving blocky looking artifacts in their place.
The internet is full of advice as to what setting you should use for what, but still the nagging question is: “Just how can you tell what kind of damage has been done to your image?”.
Below is a side by side comparison between 2 versions of the same image. The one on the left is the original, and the one on the right is a version saved as a JPEG using a setting of 5 on the quality slider in Photoshop.
Can You Tell the Difference?
Below is a tighter crop of the image side by side to make it easier to see the artifacts that show up around the guy’s head and the horizon where the sky meets the green field to his right. Again, the original is on the left while the one on the right has been saved as a JPEG with a setting of 5.
In many cases it can still be hard to see the difference. Fortunately Photoshop gives us a tool that makes it easy to see exactly where the loss of detail occurs so you can make smarter decisions about just what setting works for your images.
[Related Reading: Free Photoshop Collage Templates for Photographers!]
Comparing with the Difference Blending Mode
This tool makes use of the Difference Blending Mode. In short this Blending Mode compares the layer on top with the layers below and shows you the areas where they are different. If the layers are identical you’ll see a completely Black image, but where there are differences you’ll see lighter colors, the greater the difference the lighter the color.
To check your images you simply need to open the original version in Photoshop as well as the one you want to check for artifacts or loss of detail. Drag the layer from the JPEG version on top of the background layer for the original version so it sits right above it in the layer stack. Then change the Blending Mode to Difference. In the case of the images we’ve been using for comparison here is what we see when we do that.
If you’ve used a setting of 5 or greater when saving the JPEG, odds are the resulting image you’ll see will be mostly black as you see above. But that does not mean there is not loss of detail etc.
Use Curves to Exaggerate the Difference
To make it easier to see the difference just add a Curves Adjustment Layer on top of the layers in your layer stack and pull up on the Curve to lighten the image exaggerating the difference enough to make it easier to see.
Here is the same image using a Curves Adjustment layer to do exactly that.
See how much easier it is to see where the differences are? Now that we know where to look we can turn off the Curves layer, set the Blending Mode for the JPEG layer back to Normal and then toggle it on and off, looking in the areas where the previous test showed we should look and you’ll know just what to look for.
Repeating this test with various types of images and JPEG settings will help you quickly learn which settings you can use for which kind of image and which use will work for you.
Now that you know what to look for can you spot the difference between the 2 versions shown below?
The bottom line here is by using this simple test you can see for yourself just what detail might be getting lost when you save your images using a lossy format, such as JPEG. As artists and content creators it’s important for us all to understand just what’s going on so we can make smarter decisions for ourselves about what works and what doesn’t, especially when we have to meet our clients expectations!