The mirror filter in Affinity Photo is found in the distort category (part of the filter menu). Probably my favorite filter with excellent interactive effects and the results can be used once or multiple times to create all kinds of amazing artworks. You can use it with images and type along with other presets such as shapes and pattern layers and masks and more
It create beautiful radial / symmetry designs. By default, it displays a reflection but you can increase the number of mirrors used up to a maximum of 20. You can also change the input and output, revolving the input and output of the result which is super useful. You can also select interactively the origin point on the currently opened image.
It can be used once to create an amazing hi-impact design, you can also re-apply it over and over. It is one of those filters that multiple re-applies by using the repeat command is not that much use. You need to re-apply by going back to the panel and then set the origin, number and input and output and then apply the effect and the result of changing the origin each and every time can result in a magical image.
You can always set up a macro to do any of this via the view menu and studio and macros. Once you have it as a macro, the effect can be re-applied over and over to create all kinds of symmetry effects
You can use it in channels as well. Display the channels panel via the view menu and studio and select the red channel and then apply the mirror filter with a certain set of settings. Select the green channel and then apply the mirror filter with new settings and likewise with the blue channel.
You can apply the effect to create all kinds of amazing radial designs and then after clicking apply you can always then fade the effect as well as add a blending mode such as darken, lighten, vivid light, pen light and others. The default is set to 50% but you can always change the fade opacity
You can use this powerful effect with pattern layers. Select a layer and apply the effect. A double whammy of amazing symmetrical designs can be created and the result of the artwork can be rasterized via the layer menu and rasterize command and the effect applied again to that result.