How to find elliptic strips from a binary mask?

Question

Consider a binary mask as

Image[DiskMatrix[All,{100,150}]];

Now I wish to have a strip like this:

such that I can manipulate the inner radius, and the outer radius (here major and minor axes) without changing the original image size.

How can I do this?

Edit 1:

I just figured out that I can do this in the following way:

Manipulate[mask1 = Image[DiskMatrix[{100*m, 150*m}, {100, 150}]];
 mask2 = Image[DiskMatrix[{100*n, 150*n}, {100, 150}]];
 ImageSubtract[mask1, ImageMultiply[mask1, mask2]], {m, 0, 0.5},
 {n, 0, m}]

Is there any better way to do this?

Answer 1

If by "shrinking" you mean something like carving an n-pixel wide border from the mask, you can do this using morphological erosion with a disk structuring element

img = Image[DiskMatrix[All, {100, 150}]];    
r1 = Erosion[img, DiskMatrix[10], Padding -> 0]

r2 = Erosion[img, DiskMatrix[20], Padding -> 0]

r1 - r2

Alternatively, you can use a distance transform to calculate the euclidean distance of each point in the mask to the background:

dist = DistanceTransform[img, Padding -> 0];
ListPlot3D[ImageData[dist]]

You can then use simple algebra to select the pixels at certain distances from the border:

UnitStep[dist - 10] - UnitStep[dist - 20]

(same result)

Answer 2

Here is another way:

With[{w = 100, h = 150, m = 3, n = 2},
  Image@BitXor[
    DiskMatrix[All, m {w, h}], 
    CenterArray[DiskMatrix[All, n {w, h}], m {w, h}]
  ]
]

Answer 3

another approach. (watch, this gets real slow if you try to make it any larger)

size = {100, 200}
Manipulate[
 Image[SparseArray[{i_, j_} /; 
     a^2 < r (i - size[[1]]/2)^2 + (j - size[[2]]/2)^2 < b^2 -> 1, 
   size]],
 {{a, 10}, 0, 100},
 {{b, 50}, 0, 100},
 {{r, 2}, 1, 10}]

Answer 4

I acknowledge other answers. However, I think I am happy with this one.

Manipulate[
 mask1 = Image[DiskMatrix[{100*m[[2]], 150*m[[2]]}, {100, 150}]];
 mask2 = Image[DiskMatrix[{100*m[[1]], 150*m[[1]]}, {100, 150}]];
 ImageSubtract[mask1, ImageMultiply[mask1, mask2]],
 {m, 0, 0.5, ControlType -> IntervalSlider, Method -> "Push", 
  MinIntervalSize -> 0.01}]