How To Use the Divide Blend Mode in Affinity Photo

Using the Divide Blend Mode in Affinity Photo

In February 2021 Affinity Photo 1.9 was released with a few new features for photographers. One of the changes was the addition of a new Divide Blend Mode (which is already available in Photoshop). In this tutorial, I’ll explain how the Divide Blend Mode works. I’ll also explain how to use it to make colour corrections in your photo editing.

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What is a Blend Mode?

Blend Modes are settings we can apply in Affinity Photo to control how two things, will blend and what result they produce. Probably the most common use for blend modes is to control how two layers blend to produce a new result. You can set this in the Layers Studio Panel.

Each layer in the Layers Studio Panel has a Blend Mode assigned to it which you can set in the dropdown list at the top of the panel. By default, this is set to “Normal” for new layers, but you can produce interesting effects by changing it. Most of the blend modes have odd names like Multiple, Screen and Divide (being the latest). Whilst these might not sound helpful, they describe the mathematics behind that blend mode.

These blend modes are simple mathematical formula that determines the result of blending two pixels together. Whilst this might not sound exciting it can be extremely powerful.

Understanding the Divide Blend Mode

To understand how the new Divide blend mode works, we need to do some simple maths. Please don’t skip ahead because this is important and will help you later.

Let’s imagine we want to blend two pixels together to produce a new pixel. Let’s also imaging that these pixels are on two separate layers. On the bottom layer is a grey pixel which is 50% white and on the top layer is a white pixel which is 100% white.

We can also express these % values as regular numbers. White has a % value of 100% or 100/100 which is 1. Grey on the other hand has a % value of 50% which is 50/100 or 0.5. If we now set the blend mode for the top (white pixel) to Divide the calculation of the result looks like this.

0.5 / 1 = 0.5

In other words, there’s no change in value. This means the result of blending these two pixels together is to produce a midtone grey pixel.

You can see this in action by opening an image in Affinity Photo and adding a New Fill Layer in the Layer menu. Fill this new layer with white by selecting the white swatch in the Swatches Studio Panel.

If there isn’t a white swatch visible, click the dropdown at the top of the panel and select the “Greys” swatch option.

After adding a white fill layer to your image, select the fill layer in the Layers Studio Panel and set the Blending Mode to Divide. When you do this the fill layer will vanish and you will see the image unchanged.

Blending Darker Colours with Divide

Let’s now look at a second example. Again, imagine the same scenario with two pixels on separate layers. This time both pixels are a midtone grey which is 50% white. Now when we set the Blend Mode to Divide the calculation becomes.

0.5 / 0.5 = 1

When we divide 0.5 by 0.5 the result is 1 or 100%. This produces a white pixel.

We can then take this further by making the top pixel of this example black rather than grey. As there is no white component to a black pixel this has a value of 0%. Let’s also make the bottom pixel dark grey so it’s only 25% white or 0.25. The calculation is then.

0.25 / 0 = infinity

But because the largest output is 1 (or 100%) our pixel becomes white again.

In short, the darker the colour you use with the Divide blend mode, the lighter the result. Remember this because it will help you when picking a colour to use for colour correction.

Correcting Colours using the Divide Blend Mode

Applying a colour correction using the Divide blend mode is extremely easy. All you need to do is add a fill layer to the image using a colour from the image that should be neutral. Here are the steps:

1. To add the new fill layer, select “Layer | New Fill Layer” from the menu. This adds a new layer to the image and fills it with whatever is the active colour in Affinity Photo.
2. Select the new Fill Layer in the Layers Studio Panel and set it’s blend mode to Divide. If the layer is filled with white it will vanish. If it’s filled with another colour you will see the colour and/or the brightness of the image change.
3. Turn off the fill layer’s visibility in the layers window to hide it. You need to do this before you take a colour sample from the image. If you don’t, you will find the fill layer distorts the results.
4. Using the sample tool in the Swatches Studio panel, click and drag it over the image to find the point you want to sample. When you release the mouse button you will take a sample that then appears in the Swatches Studio Panel. See the screenshot below.
5. Ensure you have the Fill layer selected in the Layers Studio Panel and then click the colour sample you took in the Swatches Studio Panel. This applies the colour to the fill layer. When you turn the layer on to become visible, it corrects the colour cast in the image.

Whilst the steps are simple, there’s a little more you need to know to achieve good results. As you might have guessed, where you take the colour sample is critical.

Where to Sample the Image

Let’s take the example of an image which is suffering from a colour cast and needs correcting.

This image is suffering from an orange/yellow colour cast caused by the street lighting. The side of the building (point 1) should be neutral but isn’t. If you were to sample from this point you would find values of Red = 135, Green = 117, and Blue = 72 (the RGB colour channels). It’s this imbalance between the colour channels that causes the colour cast. If the three channels were all roughly equal the stonework of the building would appear as a neutral grey.

When we use this colour as the fill colour in our fill layer, the Divide blend mode will even up the channel values. Unfortunately, this sample is a dark colour so it would cause the image to become very bright as you can see here.

Instead, we need to sample from a neutral area that is much lighter, possibly somewhere around point 2.

Although you want to use a lighter colour, avoid sampling anything that’s pure white. If the sample if white (has channel values of 256, 256, 256) it won’t have any effect. It’s the imbalance between the colour channels that causes the colour cast and that’s what you need to sample.

Here’s what happens when we sample from near to point 2.

This is much better but it’s still a little too bright. We can see the light area around point 2 becoming blown out.

Taking Total Control of Colour

What the sample has done, is provided a starting point to use for the colour correction. By understanding the different ways colour is represented in Affinity Photo, we can make adjustment to achieve a better result.

Let’s start by tweaking the colour values. Perhaps you found there was nothing that was truly neutral in your image, so you need to manually change the mix of red, green, and blue used on the fill layer. You can do this in the Colour Studio Panel by selecting the RGB view.

Start by making sure you have the Fill adjustment layer selected in the Layer Studio Panel and the Divide blend mode set. With that selected, open the Colour studio panel, and ensure it’s set to display the RGB colour controls. This displays three sliders representing the three colour channels (red, green, and blue). The values of these sliders reflect the colour of the fill layer and if you change them, you change the fill layers colour.

Now look at the Red slider in the screenshot. Notice that one side of the slider shows green and the other pink. This shows you what will happen when you adjust that slider. By moving the slider to the right (towards pink) we make the image greener (the colour on the opposite end of the slider). You can adjust all three sliders to tweak the fill layer colour to achieve a neutral colour balance.

Controlling Image Brightness

Now let’s look at the problem of controlling brightness.

In the previous section we used the RGB controls to adjust the image colour. But we can also represent colours in terms of their HSL values. This can help us if the Divide blend mode is making the image too light.

Here are the controls in the Colour studio panel when you switch to the HSL model of colour.

In the HSL model of colour, the H slider (Hue) controls the colour, for example is it red, blue, pink etc. The  slider S is for saturation and adjusts the intensity of the colour. Finally, there is the L slider controlling the Luminosity or lightness element of the colour.

We already know that a darker colour will lighten an image more than a light colour when using the Divide blend mode. If you are having problems with your image becoming too light, increase the L slider. When you do this, the image brightness reduces. Unfortunately, you will probably see the original colour cast in the image return, but don’t worry.

When you increase the L element of a colour you also reduce the intensity of that colour. The way to fix this is by also increasing the S slider to increase saturation, making the colour more intense. By using the S and L sliders together you should be able to control the colour correction and image brightness. Just be sure not to change the H slider.

It’s the H slider that will significantly change the colour balance. If you want to tweak the colour balance further be sure to switch back to the RGB sliders. These give you a more granular control.

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Summary

As we’ve seen, the new Divide blend mode in Affinity Photo 1.9 is a great way to colour correct photos but it has its drawbacks. Ideally you need a light colour in the image to sample. If the sampled colour is too dark your image will become too bright. If you don’t have a suitable neutral point to sample, it can become difficult to make a correction using the Divide blend mode.

When this happens switch to adjusting the colour using the RGB and HSL controls in the Colour studio panel. These can help you create colour corrections and even introduce special effects. With a little understanding the Divide blend mode can be a useful tool in your photo editing toolbox.

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