solved merging/updating curve3

[Mark Bufton]

Hi guys,

Rather a technical question I'm afraid and one that I do have a functional solution to, but wanted to take the opportunity to learn something new.

I'm attempting to create the deck of a ship (the Titanic) from blueprints, using a curve3 object to create the outline of the decks (eventually using it to create path points for a polygon mesh/extruded shape, but simply used as line paths for the time being). This works perfectly fine, of course, as the deck plans are completely flat - using the X and Z co-ordinates (in my case) of the curve3 I can match the outline of the decks with reasonable precision.

A ship, however, doesn't tend to have flat decks - Titanic being no exception. The decks are curved upwards both forward and aft. One (almost) solution being to create an additional curve depicting the deck on a side (profile) blueprint (containing the same number of points as the aforementioned deck outline).

I have written functions to extract the x, y and z points of any given curve3 path, and could pass the x and z data from the deck curve, and the y data from the side profile curve, as arguments to my own custom curve. This would work in theory, if both curves were identical - then the x co-ordinates of the side profile would match perfectly the x co-ordinates of the deck plan's curve.

Where the poop deck, for instance, curves around sharply at the stern, the x co-ordinates of the deck outline will get considerably closer together, whereas they won't do so on the deck profile - using my 'solution' would likely result in a very warped deck.

My solution to the problem is a simple one - simply line up the two plans and visually adjust the y input of the start, end and control points of the bezier curve function (depicting the deck outline) to incorporate the curve of the deck - which is gentle enough to achieve easily.

My curious nature tends to get the better of me, as you all probably know, and leads me to ponder if there is a way to programatically get the x, y or z co-ordinates at any given point of the curve3? Take a simple curve from vector3(10,0,-10) to vector3(50,0,0) - if, for example, I wanted to know the value of z at x:31.265, am I able to get that or calculate that programatically without being a mathematical genius or including a massive number of points and searching the path array for the nearest value?

[jerome]

http://doc.babylonjs.com/how_to/how_to_use_curve3

Unfortunately, there's no way to get a value from a curve at some distance or a given x value. Why ? because, despite it's called a "curve", it's actually a series of points (Vector3) linked by straight linear segments. All these tiny segments rendered together are an approximation of the real mathematical curve.

So given some x value, you could maybe get the nearest surrounding points V1, V2 and interpolate the right y on the line V1V2. 

[JohnK]

The following PG uses the start, end and two control points needed by Curve3 to draw a cubic Bezier Curve to produce a function that returns any point on the curve given the fractional distance (t) along the curve.

https://www.babylonjs-playground.com/#CCTRHM

Hope it helps

[Mark Bufton]

Lovely. Thanks guys!!!