Masking an image with an annulus

Question

I have an image where I would like to do 2 things:

a) Put a disk of radius r on an specific $\{x,y\}$ position on the image such as the result is the original image within the disk and black color outside.

My attempt was this, but the resulting image within the disk looks different

 i = Import["http://exampledata.wolfram.com/coneflower.jpg"]
 DiskRec[x_, y_, rr_] := Graphics@Disk[{x, y}, rr];
 HighlightImage[i, DiskRec[45, 40, 30], Method -> "Brightness"];
 ImageSubtract[i,HighlightImage[i, DiskRec[45, 40, 30], Method -> "Brightness"]]

b)Then, I would like to draw a black circle within the previous one, with a smaller.

Thanks in advance, Juan.

Answer 1

Get the image:

i = Import["http://exampledata.wolfram.com/coneflower.jpg"];

Define regions for: the whole image, the window disk, the black dot disk:

imRegion = Rectangle[{0, 0}, ImageDimensions[i]]
disk1 = Disk[{80, 60}, 40];
disk2 = Disk[{80, 60}, 10];

Window by changing everything in imRegion but not in disk1 Black:

windowed = ImageApply[{0, 0, 0} &, i, Masking -> RegionDifference[imRegion, disk1]]

Dot by changing everything in disk2 to Black:

dotted = ImageApply[{0, 0, 0} &, windowed, Masking -> disk2]

Done knowing what result you want before starting, we can just skip to defining your region of interest as an Annulus:

roi = Annulus[{80, 60}, {10, 40}];
viewRoI = ImageApply[{0, 0, 0} &, i, Masking -> RegionDifference[imRegion, roi]]

This returns the same as the dotted version of windowed, so I won't add another image.

EDIT:

There was a request to move away from using Region, because so much of Region support is new (and not implemented in old versions).

We can use ImageApplyIndexed to achieve this effect as long as we keep the "Regions" of interest simple enough that it is easy to test if a coordinate pair lies in the region.

Our Version 10 - robust method looks like this:

Define an image:

i = RandomImage[1, {150, 113}, ColorSpace -> "RGB"];

Define some parameters to characterize the disks:

{d1Center, d1Radius} = {{50, 80}, 40};
{d2Center, d2Radius} = {{50, 80}, 10};

- Note that because of how ImageApplyIndexed feeds pixel coordinates, the center here is specified as {"row from top", "column from left"}.

Now we turn all pixels not in disk 1 Black:

windowed = ImageApplyIndexed[If[EuclideanDistance[#2, d1Center] > d1Radius, {0, 0, 0}, #1] &, i]

And then turn just the pixels in disk2 Black:

dotted = ImageApplyIndexed[If[EuclideanDistance[#2, d2Center] > d2Radius, #1, {0, 0, 0}] &, windowed]

Answer 2

We can define a general mask based on Annulus[]. Then Rasterize the Graphics, resize it, padd it, and locate in any position in the image to be masked:

mask[img_, pos_, rin_, size_] := ImagePad[ImageResize[
ColorNegate@
Rasterize@
 Graphics[Annulus[{0, 0}, {rin, 1}], 
  PlotRangePadding -> 0], {size}], {{pos[[2]] - size/2, 
img[[1]] - (pos[[2]] + size/2)}, {img[[2]] - (pos[[1]] + size/2), 
pos[[1]] - size/2}}]

As an example the mask resulting is:

m = mask[ImageDimensions[i], {50, 50}, 0.2, 50]

and applied to the image provided:

ImageMultiply[i, m]

Another example:

i = Import[
"C:\\Users\\Public\\Pictures\\Sample Pictures\\Penguins.jpg"]

m = mask[ImageDimensions[i], {500, 500}, 0.5, 500];
ImageMultiply[i, m]

Further improvement would consist in taking into account that the annulus will always be inside the image dimensions.

EDIT

When Annulus is not available, this also works:

mask[img_, pos_, rin_, size_] := ImagePad[ImageResize[
ColorNegate@Rasterize@
 Graphics[{Disk[], White, Disk[{0, 0}, rin]}, 
  PlotRangePadding -> 0], {size}], {{pos[[2]] - size/2, 
img[[1]] - (pos[[2]] + size/2)}, {img[[2]] - (pos[[1]] + size/2), 
pos[[1]] - size/2}}]

Answer 3

thanks to your input I was able to find an answer for my old version.

 i = Import["http://exampledata.wolfram.com/coneflower.jpg"];

 dim = ImageDimensions[i];

 R[x_, y_, rin_] := RegionPlot[RegionDifference[Disk[{0, 0}, Max[dim]],Disk[{x, y}, rin]], Frame -> False, PlotStyle -> {Black, Opacity[1]}]

 mask[img_, pos_, size_, x_, y_, rin_] :=ImagePad[ImageResize[ColorNegate@Rasterize@Graphics[R[x, y, rin],  PlotRangePadding -> 0], {size}], {{pos[[2]] - size/2,img[[1]] - (pos[[2]] + size/2)}, {img[[2]] - (pos[[1]] + size/2),pos[[1]] - size/2}}]

 Manipulate[{R[xx, yy, rre],m = mask[dim, {dim[[2]] - a, b}, rre, xx, yy, rr],multi = ImageMultiply[i, m]},{a, 1, dim[[2]], 1}, {b, 1, dim[[1]], 1}, {rre, 1, dim[[1]], 1}, {xx,1,dim[[2]], 1}, {yy, 1, dim[[1]], 1}, {rr, 1, dim[[1]], 1}]