va2m

Can we expect a way to "undo" the "Fit to plane" operation applied to an object? And not only by going Ctrl + Z, but for any selected object. Or maybe it's there but I am missing it.
Thanks in advance.
Best regards!

Here is the problem.  Our current parallel axonometric system allows for non-destructive transformations as the inverse is easy to determine.  The transformation is also just a simple shear-scale-rotation applied to the original object.  The underlying geometry itself remains unchanged.
We also use affine transformations for our document model, allowing us to deduce an inverse.
 
Converging perspective requires deformation of the original geometry.  Anything nearing the vanishing point will tend towards a size of 0.  It is impossible to then determine an inverse transform for anything that has been transformed in this way.  We do not currently preserve any notion of "original geometry" as such.  Only, a transform for which we can return to a 2D representation.
 
If we do converging perspective, it will have to use destructive deformations to stay in keeping with our current Document Model.  This is not a 3D application - it's a strict 2D application.  Any notion of 3D (or 2.5D) is purely illusion.  I've reiterated this point many, many times.  In order to provide a full perspective model a full concept of 3D position is required.  We are not going there.
 

A few days ago I used the isometric perspective tool to make a mockup of a board game. The result is not bad, but with convergent perspective would be much better.
I have created the mockup in a separate document in which I copy and paste from other files the 2D elements of the game (map, cards, counters...), then pass them to isometric perspective using the function fit to plane.
For me, the convergent perspective would be very useful even if it was destructive.
Does anyone else find it interesting, is there any possibility of having this function in the future?

Yes - at some point.  There will be some improvements to axonometric tools.

Convincing 2D converging perspective is not that simple. If it were, we wouldn't need any special features for it. We can already just draw arbitrary horizon lines and perspective rays like we do on paper.
Nor do I assume it's that simple to implement in software. Have you tried the Perspective Grid feature in Adobe Illustrator (which it copied from FreeHand)? Many users no doubt think it must be rigorous just because it's in software. But it's easy to catch it committing the common error in which foreground grid "squares" become less foreshortened vertically than horizontally and therefore look like elongated rectangles instead of squares (not at all how the eye would see it).
Here's the thing: The kind of casual "vanishing point" perspective typically taught in basic art classes is actually not very rigorous. Students get exposed to so-called "one-point", "two-point", and "three-point" perspective and are led to assume that three-point perspective is some kind of end-all of ultimate realism just because the subjects we draw reside in 3D space. It's not. A view of 3D objects in 3D space can have an infinite number of "vanishing points." (Think of tossing your toddler's collection of building blocks upward into the air and taking a photo.)
Yet just watch a few of the many amateur YouTube videos on the subject and listen to how many times you hear the demonstrators use phrases like "There you have it; a perfect perspective of [this or that]!" Also note that they typically just place their vanishing points at any random locations relative to each other, without any kind of actual geometric basis.
Rigorous converging perspective must be based on the geometry of optics; of conic vision. Grid squares don't merely shrink toward the distance, they change in shape. (I wonder how many graphic designers and even illustrators understand that if you take a photo with a "normal lens," crop it down to a distant part of the subject, and then enlarge it, the resulting image looks like it was taken with a more "telephoto" lens.)
There are more formalized methods of constructing converging perspective. For example, before computers architects used a more elaborate construction method which did a better job of constraining things to realistic proportions. Technical illustrators had special equipment like Klok Perspective Boards and Tables and their graduated ruler scales.
That's not to say that the willy-nilly kind of informal converging perspective is "illegitimate" as an illustrative style. It's not. Grossly exaggerated perspective is commonly used just to add overstated drama in everything from Marvel's super hero comic books to the Marketing Department's rendition of the corporate trade show booth.
My point is that "vanishing point perspective" is a broad subject encompassing methods and purposes that range from rigorous to whimsical. So a "good" software implementation of it is an ill-defined target.
Axonometric drawing, by comparison, is not like that at all. There's nothing ambiguous or arbitrary about it. It is either geometrically correct or it is not, in fact, axonometric.
Again, the Lazy Nezumi Pro approach to interactive drawing aids causes me to re-think the need for program-native features for converging perspective. For example, one of the demo video clips on the LNP site demonstrates its Fisheye perspective variant. How often have you ever seen that kind of 2D perspective convincingly executed by the traditional "vanishing point" methods? That novel implementation alone represents a lot of potential.
In my previous post, I mentioned that the applicability of LNP to vector-based drawing is largely dependent upon the number and quality of Bezier path drawing tools that are used by dragging along the onscreen guides. It may be more advantageous for Affinity development to simply focus on optimized Bezier path tools that are of superior quality in terms of functional economy and editing elegance; i.e., the number and placement of nodes and handles automatically generated when the tool is dragged.
JET

Meanwhile, as Frozen mentioned, to those who haven't yet, it really is worth looking at Lazy Nezumi Pro.
LPN's claim-to-fame is that it is not an "add-on" for any particular graphics program.  It sort of gives your computer some drawing smarts, instead of just a particular graphics program. The drawing or painting program you're using doesn't even know it's there. You just launch it and then click the application window that you want to work within. It works by affecting cursor movement at the system level, "smoothing" your pointing device movements to constrain to the on-screen guides that it effectively "overlays" within that window. It's the closest thing I've seen to software actually emulating the use of physical drawing aids like rotatable rulers and ellipse templates. The way it works gives it these huge advantages:
Although it has a quite sophisticated collection of setup options, it's worth the learning curve because you learn it once and it works the same way regardless of what drawing or painting program you're using at the moment.
  Although originally presented as a drawing aid for raster-based drawing, it's not just for raster imaging. It's just as applicable in windows of vector-drawing applications; it just depends on your having selected an appropriate drawing tool. Yeah, it works with path drawing tools you think of as "freehand" (pencil, brush, etc.), but it's just as useful for a line tool (or Affinity's Pen Tool in Line Mode). It's basically appropriate for any tool you drag along the desired direction because that's what it does; it affects the smoothness or straightness of your drag inputs.
  Its perspective features include configurations for constructing both converging perspective and parallel perspective.
  Contrary to common misconception, it is not just for using a stylus (which I very seldom use). On desktops or laptops, it doesn't know or care what you are using for a pointing device. (I can't speak to the matter of finger gestures on cell phones because I quit finger painting before kindergarten. So LNP is not something that competes with Affinity, it just serves as a drawing enhancement for it or whatever graphics program(s) you are using. I think you can download it as a demo to try it out in whatever program(s) you have in mind.
JET

I've thought about vanishing perspective tools - and as @JET_Affinity says, they will have to be completely separate to our standard grids and planes system.  From a 2D construction point of view, there is a lot more can be easily determined or inferred in parallel systems.  Converging perspective needs distortion - and to preserve any notion of the plane an object lives in is more involved (or even impossible without over-complicating the tools).
 
I will be working more on snapping, grids and custom guides soon.  It may well fall into that work, but I am making no promises.

The kind of "vanishing point" converging perspective va2m is describing is entirely based on 2D construction. And it's nothing new. Macromedia FreeHand implemented a grid system for it way before it was copy-catted into Adobe Illustrator (after Adobe's acquisition of Macromedia).
But I'd suspect it would be cleaner (both from the programming perspective and for the user interface) to implement it separately from the parallel perspective grids feature still under development in Affinity.
JET

Desktop_2019_07.15_-_22_13_18_02.mp4 Lazy Nezumi Pro does the thing va2m is asking for and it does not rely on 3D support to work, or a grid for that matter. Heck, you can even load that add-on manually in the Affinity Suite to make all its perspective tools work, although it's slightly buggy since it doesn't fully support Affinity, and will not be supported as long as the programs lack high precision tablet input (which will be added at some point according the the devs). These perspective tools rely on mathematical equations to fake the perspectives in a non-3D environment. It's not really that different from the already existing perspective transform tools in Photo, which are also not real 3D.
Above is a quick video I recorded showing how this add-on fakes perspective when loading it into Affinity.

Affinity Designer already has a grid and axis manager, to select diferente types of grids, like isometric, dimetric, ….
Also it also has the isometric panel, that change planes to edit or fit objects in a given plane.
A new option of grid could be developed to do perspective drawings, like on paper with One Point Perspective or Two Point Perspective, that uses vanishing points to create the illusion of depth in an drawing. And probably using a similar organization of isometric drawings.
This use of the geometry in the grid and panel, would be very useful for to make this type of drawings.
Now, we need to create lines to achieve this, and it is not practical or productive.
Does anyone also finds this useful?
Cheers