The perspective filter in Affinity Photo can be found in the distort menu beneath the filters menu. The perspective effect is a wonderful powerful distortion tool that can distort any image in many ways in single plane or duo plane mode. Use images as well as type and also resources such as patterns and brush strokes
Run the effect. Grid appears as well as panel. Select planes and mode. Move grid as required using the displayed control points (little square block points) OK, there is a lot more to the filter such as the source mode and auto clip as well as two types of planes (single and dual plane) and you can also use it to create subtle shifts of an image as well as truly extreme distortions, really depends on what you want to do such as add an object / layer in the same way as the background.
You can really create some weird distortions / extreme distortions with the single plane (as well as the dual). It can be applied multiple times but you should realize that it is a destructive effect when using the filter and the only way to return to the original image is to use the history panel or undo. Open image and filter menu and distort category and apply. Go to the top left corner of the warp grid. select the control point. Drag close to the bottom right corner (not all the way but fairly close) distorting the grid (and overlapping the image) - the key thing is to make certain you still have some of the original image still there otherwise all you will get is stretched pixels from the edge. Press return and repeat. Perhaps choose the top right corner control point this time. Repeat distortion. Press return. Repeat
The dual plane option can be selected instead of the single plane. With the dual plane you have an additional set of control points to manipulate (center). You can use the tool to distort images but you can also use the tool to add perspective and alignment to perhaps buildings and text and other objects in Affinity Photo. Go to the panel for the filter Set to dual plane. Go to the control points in the center and move up or down or left and right. Go to the control points at the far ends of grid and distort as well. Press return. You can create all kinds of unusual distortions such as a mirror effect with an image
You can mirror (and distort) the artwork using the dual plane option. Apply effect. Dual plane. Go to the left control points. Select bottom left control point and move above the top left control point. Move the previous top left point further down below the previous bottom left point. Drag the left points off the screen. Move the center points to position the 'mirror'. You can create all number of variants of this twist and you can also repeat the effect again and again to create even more extreme effects
The tool is also available via the toolbar with the same functionality as the filter. As soon as you select that tool in the toolbar, you will see the panel which can then be used to manipulate the grid (it is easier if you use the show grid option)
You can use the filter layer as well as the normal filters with layers. You can add subtle shifts of perspective to a layer / element in an image such as distorting a house etc to match another house in the image background. You can use it also for interesting and more extreme collages of images and layers. Open image and then place an image via the file menu place command and then apply the distortion effect to the new placed layer (single or dual plane). Distort the new layer (which may be the same image or not). Repeat with a new layer and so on. This can be done with a couple of layers as well as many layers. The great thing about the layers is that they can be used with blending modes as well to create even more interesting color effects as well as adding masks and layer effects into the mix.
With the single plane, you can move the four corners which by default fill the entire image or layer. With dual plane, you can move the four corners as well as the central line (two points) but sadly there is no rotation ability for the line though the four corners can be twisted over themselves to create some truly extreme distortion of images. All kinds of extreme distortion can be created as well as subtle changes in both plane types. Personally, I like to work on a layer as you can see the original image behind the distortion in the left image (it is a dual plane image with the left half hardly distorted and the right half stretched to the near limits) You can also use the single as well as the dual (but I think the single works best for basic transforms) just to add a subtle scaling of a layer as well as add some distortion and it can be combined with layer effects as well. To have the image slightly shift into the distance, as a plane on the ground. Select image and duplicate layer and use the effect and single plane. Shift the corners down into the bottom half of the image and slightly distort. Use the effect menu and add an outer shadow
Probably the oddest, the destination mode gives a more visual feedback as the grid moves as you move the points. With the source option, you can't see anything but the results can be even more dramatic (though perhaps not so predictable). As with all effects, you can re-apply the effect either using different settings or just the same settings via the filter menu and repeat. The repeat with the source as well as destination mode can result in even more extreme image effects with the distortion fragmenting the original image.
You don't have to use the entire layer, you can use the effect with a selection. Open image and go to the elliptical marquee tool in toolbar and apply the effect to that selected area and press return. You can now distort just the selection part. Weirdly, you can create interesting echo or fragmented effects with the destination mode