Subset Games Forum

Could someone explain how a ship's base, cloak, floor, and shield images are laid out?


kestral.xml
kestral.txt
The wiki isn't clear on what each of the x/y offsets are relative to and how the images' bounds affect one another.

The cloak image is always scaled 20 pixels wider and taller than the base image, so presumably it's positioned at baseX-10, baseY-10 to keep their centers lined up?

The floor's position/scale wasn't mentioned at all (Given the 35px sprites, there shouldn't be any floor scaling).

These are described as offsetting the 'entire' ship.
X_OFFSET*35 + HORIZONTAL
Y_OFFSET*35 + VERTICAL

The Kestral's base seems to be centered vertically on the shield image, so what's that y="-116" doing where the wiki says it "Slides the base image"?

From what I worked out while developing Superluminal...

FTL uses positioning system with X axis increasing rightwards, and Y axis increasing downwards:

• X axis: positive - right, negative - left
• Y axis: positive - down, negative - up

As a consequence, objects are anchored in their top left corner, ie. object placed at point (0, 0) with size (width, height) will have the other three corners at points (width, 0), (0, height), (width, height).

Ship's origin point - Po

• FTL has a predetermined, hardcoded common point (separate for player/enemy ships) that serves as origin for further translations.
• X_OFFSET translates the point by #*35 pixels in X axis
• Y_OFFSET translates the point by #*35 pixels in Y axis
• HORIZONTAL translates the point by # pixels, X axis
• VERTICAL translates the point by # pixels, Y axis

Let's call the resulting point Po
Rooms and doors are then placed relative to Po.


Images' origin point - Pi

• Ship images use Po as origin for further translations
• <img>'s x= attribute translates the point by # pixels, X axis
• <img>'s y= attribute translates the point by # pixels, Y axis

Let's call the resulting point Pi.

Ship image properties:

• Base, floor, cloak, gibs are all placed at Pi
  [cloak having x - 10, y - 10]
• <img>'s w= and h= attributes determine the size of base, floor and cloak
  [cloak having width + 20, height + 20]
• Gibs are further translated from Pi on an individual basis via <gib#> tags' <x> and <y> tags

Shield image origin point - Ps
I explained that not so long ago here, but I guess it's pretty confusing, maybe this image will clear it up:

Red rectangles - rooms
Blue rectangle - bounding rectangle - smallest rect containing all rooms

Ps is placed at the intersection of the bounding rectangle's diagonals. It doesn't have to line up with the room grid (could be in the middle of a room, for example)

Ellipse - shield image and collision perimeter The ellipse is then calculated using the following equation

Ps,x is the x component of Ps
Ps,y is the y component of Ps

What this basically means, is an ellipse with width = 2*a and height = 2*b, centered at Ps offset by the x and y offset values.

The resulting ellipse is the projectile collision perimiter, defense drone orbit, and shield.
For whatever reason, player ships' shield image is not affected by the ellipse's width and height, only the offset values.
However, enemy ships' shield image is resized to match the ellipse exactly.


...soo yeah, that about covers it, I guess

kartoFlane wrote:Ps is placed at the intersection of the bounding rectangle's diagonals.

The bounding rectangle of the ellipse being double the axes' lengths.

So for the Kestral, centering a 700x440 shield image at (Pi+350-30, Pi+220+0).

And the the y="-116" is shifting Pi - and all the images - upward while leaving the rooms in place?


I'd forgotten about that gap between the room grid and the top edge of the hull.

Vhati wrote:So for the Kestral, centering a 700x440 shield image at (Pi+350-30, Pi+220+0).

And the the y="-116" is shifting Pi - and all the images - upward while leaving the rooms in place?

Precisely