How to Use the Selection Brush Tool in Affinity Photo

by Dec 8, 2021Photo Editing Tutorials

Robin Whalley Landscape Photographer

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How to Use the Selection Brush Tool in Affinity Photo

A common photo editing task is selecting an area of a photo to adjust, without affecting the rest of the image. Often, it’s not the adjustment you apply but how well you make a selection that determines the quality of your image.  In this tutorial, I’m going to explain how to use the features of the Selection Brush Tool in Affinity Photo to create great selections.

Simple Selection Brush Example

You will find the Selection Brush Tool in the Affinity Photo Tools Palette on the left of the interface.

Selection Brush Tool in Affinity Photo

Click the icon for the Selection Brush Tool with your mouse to activate it.

You’ll then see the context sensitive toolbar display the options for the Selection Brush. We will look at these shortly, but for this example we will use the defaults.

With the Selection Brush tool active, position the mouse pointer over the area of the image you would like to select. Then click and whilst holding the mouse button down, drag it over the area to select. As you move your mouse pointer, Affinity Photo selects where you paint.

The Snap to Edges Option

One extremely useful feature of the Selection Brush Tool is the Snap to Edges option. You find this in the context sensitive toolbar along the top of the interface. Click the option to toggle it on and off.

When you use the Snap to Edges option, Affinity Photo checks the area around where you’re painting. When it finds an obvious edge in the image, it extends the selection to meet that edge. This can save a significant amount of time and reduces the brush strokes required for the selection. More importantly, it creates an extremely accurate selection that you couldn’t achieve by painting alone.

Viewing the Selection

In this example I placed the mouse pointer over the top left corner of the image. I then clicked and dragged to the right to select the sky. This creates an extremely accurate selection of the sky, down to the horizon where it meets the mountains.

Selection of the sky created using the Affinity Photo Selection Brush Tool

You can see the selection marked on the image by and dotted line. This dotted line is animated to make it easier to see and is often referred to as “marching ants”. An alternative view that makes it easy to see the selected area is the mask view. You can activate the mask by clicking the small icon in the toolbar, indicated by the arrow in the screenshot above.

Selection showing the mask view in affinity photo

After clicking the Mask icon, a red mask overlay covers the area of the image that isn’t selected. You can toggle this by clicking the icon in the toolbar again, which switches between the mask and the marching ants view.

If you don’t like the red mask, you can display it in black, white or as a transparent area. To do this click the small arrow to the right of the icon to display a drop-down menu.

To clear a selection, click Select in the Affinity Photo main menu and then the Deselect option. An easy alternative is to use the keyboard shortcut Cmd + D (on a Mac), or if you’re using a PC Ctrl + D. If you remove a selection before you’re finished with it, you can reselect it using Select and then Reselect in the main menu.

Hidden Behaviour of the Selection Brush Tool

If you’ve already been trying to use the Selection Brush Tool in Affinity Photo,  you may have experienced varying degrees of success. Sometimes the brush works great whilst other times it might not work well at all. If you experience this, you may have decided the problem is the photo you are working on, but it’s quite possible it was something else.

Something that isn’t widely publicised is that the sensitivity of the Selection Brush Tool varies depending on its size. If you make the brush larger, Affinity is more aggressive making the selection, often selecting larger areas of the image. If you make the brush too large, you find that it selects large areas of the image rather than the area you are painting over. If you find it difficult to make an accurate selection, try reducing the size of your brush.

You can change the size of the Selection Brush in the context sensitive toolbar using the width control slider.

Changing the size of the selection brush tool in affinity photo

Alternatively, you can use the square bracket keys on your keyboard. The left [ key reduces the size of the brush whilst the right ] key increases it.

The All Layers option

The All Layers option allows you to use the Selection Brush Tool to work with all the layers in an image at once or work with individual layers.

Selection brush used without the All Layers option

In this example I’ve created an image from two images on separate layers. When I turn off the All Layers option in the toolbar, I can make a selection using either but not both of the layers. The other layer is then ignored as you can see from the mask view.

When the All Layers option is active, trying to select the sky from the background layer creates the following selection.

Selection made using the all layers option

This time when I try to select the sky the Selection Brush Tool identifies the edge of the second layer. In other words, it’s sampling all the layers to make the selection.

The Soft Edges Option

The Soft Edges Option applies anti-aliasing along the detected edges. This creates a softer edge which helps to blend any adjustments you make. Without the anti-aliasing the edge would appear jagged when you zoom in.

Soft edges option compared

Here you can see the same section of the image magnified to 400%. The left side shows the selection created with the Soft Edges option disabled. On the right is the same selection made using the Soft Edges option. Notice how the anti-aliasing produces a smooth edge that would be less easy to detect following adjustments.

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Summary of How to Use the Selection Brush Tool

I’ve published an accompanying video explaining the Affinity Photo Selection Brush Tool which you can watch on my YouTube site.

In this tutorial, we’ve looked at the different options you can use with the Selection Brush Tool in Affinity Photo. One of the most important is the way the brush behaves, depending on it size. If you’re struggling to make accurate selection, be sure to try resizing the brush.

Whilst the Selection Brush Tool is extremely useful, it has its limitations. For example, you can’t use it to make a complex selection like the sky in this image.

Complex selection using the selection brush tool

Notice how the Selection Brush attempts to select the sky but it can’t handle the fine detail of the tree branches. It also can’t select the areas of sky appearing between the tree branches. For a complex selection like this, you would need to use something like the flood fill tool and the Refine option. If you’d like to know how to do that, read this article next.

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