The deform filter in Affinity Photo can be used to distort and transform images, type, shapes etc. It in Affinity Photo can be found in the distort sub menu of the filters menu. It is one of my favorite tools for modifying images as well as other resources in the application.
As with all effects, it needs something to work on, a pixel layer or a shape (converted) or text. Go to the filters menu and distort and then select the command to display the panel (there is not much on the panel though). Add handles Drag the handles and this will distort the image. You can have extreme distortion by setting the master to 100% or set the value low for very subtle shifts of a design, perhaps to slightly move a lamp post or change the position of a hair and set it to 1% etc. You can also set the constraints to the default rigid (my least favorite) and similarity (great for dragging the underlying image around such as colors in a gradient) and there is a massive difference between the two
There are very few initial options in the panel : master and rigid / similarity constraint The difference between rigid and similarity can be subtle and then with other images and applications of handles, the result can be quite dramatic and it depends on the underlying image and selection of the pixels. The actual functionality comes from clicking the image or layer to add a handle and then select and then moving the handle as well as adding additional handles and again selecting and moving the handles. The tool doesn't appear to work at first if you only add one handle, it does need two or more handles to work.
You can always set it to 5% or 20% etc, create a subtle shift of a design instead of an extreme one. You can see by the use of the handles and pins that the text is only slightly shifted with a low setting, instead of using the default 100% setting.
Say you don't want the edge to warp, sadly there is no handle edges. You can use the handles to do the same by just adding loads of them around the edge and not touching them afterward such as dragging the handle. You can then manipulate the center of the image using the tool by adding handles to the rest of the image. There may be a slight movement of the edge but if you add lots and lots of handles around the edge, it should be slight (it is a pity that the app has no force fix edge feature)
You can also add them in multiple numbers around a particular object to freeze that part of the image and then distort all the rest. The bin in the picture (or trash can !) has handles all around it and they have not be dragged in any way. The rest of the image has been warped as usual using the dragging of the handles.
Add two pins away from the area you want to pinch and then move the handles towards the object such as a gradient line.
Add a pin or two around a text layer and then drag the handles and you can distort the text. I think the result always looks a little coarse but you can always combine it with a smooth Gaussian blur beforehand and then use the distortion to create a much more interesting effect. Even more effective is if you create multiple copies of the text layer and apply a blur to them but have different colored text with the same words. Use the layers panel to select the text layer to distort. Great for creating the weirdest shadow effects with text. As the design has been warped over multiple layers, you can also add in blending modes via layers such as linear burn.
As with the type / text layers, you can also use the effect to distort and deform a shape layer such as a star or polygon layer or cloud shape. It is a destructive option so you cannot then change the number of points in a star etc after the application. You can select all around the edge of the shape (such as a star) and then add a handle to the center or outside the shape and then drag and distort that artwork. You can also simply add a couple of handles and drag those to distort a star or shape layer in all kinds of ways. Once you have applied the the effect, it can be re-applied. As with the text, I often find a slight blurring of the shape creates a nicer result.
One frustrating thing with the effect (as well as others) is that there is no blending mode attached so if you want to add in some blending with other layers as well as the original artwork then the only way is to use the duplicate command (or use the artwork on a layer if being distorted on top of an image) and then use the blending mode in the layer panel (as well as opacity). Open image and layer menu and duplicate and then change blending mode for the top layer such as to difference etc and go to the effect again
You can fade it via the layer menu and fade. It is a great way to combine the original image with the warped design using opacity as well as blending modes such as lighten or darken to create all kinds of amazing variants https://youtu.be/SiFQ3abvyiI
You can always simply re-apply the it on an image or layer or shape etc and you can distort the designs in countless ways by using that. The repeat command is at the top of the filter menu. Great for creating metallic or molten effects. Repeat are also great for creating weird and wonderful layer distortions and the results can be quite unpredictable https://youtu.be/Ba9ZK71tsCQ
It can be captured in a recording via the view menu studio macro panel and you can then repeat the warping with any design simply by running the macro, you can also combine multiple others with it in the macro. The macro can also be saved to the library panel for future work.
You can use it with channels so go to the view menu and studio and channels to display the panel. Select the red channel and then apply the effect to that with one set of pins and handles and repeat for green etc. You can create all kinds of amazing color combinations by using the color channels feature
You can use the selection tools to select parts of the image, use add to add to the selection for the freehand selection tool etc. Once you have your selection, you can feather it and then go to the menu and apply the command. You can drag from outside as well as inside the selection but changes will happen only to within the selection. As before, you can fade it as well as re-apply. https://youtu.be/-OiXVcZQGGM