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Splitting the image

Splitting the image - Images with large amounts of colour noise can lead to bigger files when using Jpeg compression, Matthew Page describes his technique for reducing the noise, without sacrificing detail.

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Category : Adobe Photoshop
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Images with large amounts of colour noise can lead to bigger files when using Jpeg compression, Matthew Page describes his technique for reducing the noise, without sacrificing detail. Photos by Nic Duncan

There are times when splitting an image into its component parts is extremely useful. Hopefully through this little tutorial you will get an appreciation for layers, layer blend modes, and luminosity and colour components.

First of all, a quick primer about image components. You can represent an image in a number of ways. You are probably already aware that you can describe the image pixels in Red, Green, and Blue (RGB) quatities.

However, there are a number of other ways to represent the image information. The JPG picture format actually splits the image into 2 separate components – luminosity and colour.

The Luminosity is, if you like, the grey scale part of the image. (My tutorial on “Colour Punch” described how to get perceptual black and white images easily. This is the luminosity content).

The Colour part contains hue and saturation together, and can look very strange on its own. However, being able to manipulate the colour part can be extremely helpful in reducing colour noise in an image.
Colour Noise and JPG

The JPG format records the grey scale part almost as-is. The colour part, though, is run through a complex mathematical process where the high frequency information is thrown away (hoping you don’t notice it!) so the image size can be reduced.

When an image contains either a lot of colour information, or a lot of colour noise, squishing down the JPG file can be very difficult – you find you cannot reduce the file to a specific size (e.g. 200k!) without losing critical image information.

To help the JPG encoding along, we can tackle the colour information manually and make the encoding process easier – thereby gaining some file space (particularly important where file size restrictions are in place on certain websites!).

A Sample Image
Here’s an image (thanks to Nic_WA) that contains a great deal of colour noise:
And here’s a 200% view:

You can see the noise in the skin tones.

Here’s the process to split out the colour component of the image:

First, add a new blank layer
Now fill the layer with 50% grey (Edit / Fill)
Now change the layer blend mode to “Luminosity”
You should now see the colour information… but we need to “preserve” that information in a layer, so:

Hold down the ALT key and select the menu “Layer / Merge Visible” (Alt-Ctrl-Shift-E). A new layer will appear (I’ve named it “Colour component” so you can see what’s going on.
Before we ditch the grey layer, we have one more job for it.
Hide the “Colour component” layer (by clicking on the little eye in the layers palette)

Change the blend mode for Layer1 to “Colour”
Now you should be able to see the black and white image information. Once again…

Hold down the ALT key and select from the menu “Layer / Merge Visible” (again I’ve renamed the layer so you can see what’s happening)

You can now delete the “Layer 1” layer as it has done its work.
Now, that’s done the image split. We have a separated colour component, and a separated luminosity component. It’s time to filter the colour component.

Filtering the Colour
For this little trick I’m going to take you into an interesting world of other filters! There is a very useful filter called “High Pass” that keeps the detail in an image and throws away the non-detail. You may have heard of  “High Pass Sharpening” as a general technique, but I’m going to show you “High Pass Smoothing” as an alternative.

Duplicate the “Colour component” layer (select the top layer and press Ctrl-J is a good way)

Now call up the menu: “Filter / Other / High Pass”
Here you need to pick a radius that just shows the details you want to get rid of. For this 1500x1000 image I chose 2.5 – it will be image dependent. If you go too large on the radius you will throw away too much colour information and colours will bleed in the final stages.
      You should see a mainly grey image with feint patches of colour detail.
Now change the blend mode to “Overlay” (I’ve renamed the layer to “Denoise” to help)

Woah! More noise! Loads more in fact. We have emphasised the noise (this is high pass sharpening) but...

Now invert the “Denoise” layer (Ctrl-I) and voilá! The noise disappears!
The final thing to do is to group the “Denoise” layer so it only affects the “Colour Component” layer. Press Ctrl-Alt-G (or the menu “Layer / Create Clipping Mask” – in CS2)

So, at this stage we have a Luminosity component, and a filtered colour component.

Putting it back together
To re-apply the colour information to the luminosity and glue it all together:

Select the “Colour Component” layer

Change the layer blend mode to “Colour” (from Normal)

And that’s the image back!
If you can see a difference, then you’ve gone too far. However, when you save for web you should find a reasonable difference between the compression ratio you need to achieve a fixed JPG file size.

Here’s that 200% closeup after denoising:

Less skin noise for sure, and the save for web could achieve a higher quality for the same filesize.

For my last trick
Since we have split the colour information we can also adjust saturation in a non-destructive way.

By adding a grouped-layer set to “Saturation” blend mode:

Now, pick a soft brush set to 10% opacity and:
  • Paint black to reduce saturation
  • Paint bright red to increase saturation

You can, of course turn off that layer to compare. And, you could also re-colour the original image by painting on a layer set to “Colour” blend mode.

The same principles apply to modifying the Luminosity component – you can make changes without affecting the colour (hue & saturation).

Aren’t layers wonderful?!

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