This solution looks scarier than I was hoping, but you can use EditableImage:WritePixels() to overwrite the alpha component (opacity) of pixels in a certain region. However, this region is rectangular, so we need a little math to get things circular.
You can paste this code directly into the command line to produce the example shown above. It essentially draws the translucent background and cuts out the circle (which is stretched to fit the screen).
-- Conjure a magical function to cut out a circle
local function CutCircle(image: EditableImage, position: Vector2, radius: number, alpha: number)
local center = image.Size/2 + Vector2.new(0.5, 0.5)
local diameter = radius * 2
local pixels = table.create(diameter^2 * 4, 0) -- Initialize a square of transparent pixels (r, g, b, a)
local x0, y0 = position.X, position.Y
-- Keep pixels outside of the given radius translucent
for x = 1, diameter do
for y = 1, diameter do
if (center - Vector2.new(x + x0, y + y0)).Magnitude > radius then
pixels[((y - 1) * diameter + x) * 4] = alpha -- Set alpha component of each pixel
end
end
end
-- Overwrite the image region
image:WritePixels(position, Vector2.new(diameter, diameter), pixels)
end
local alpha = 0.5 -- Background opacity (1 = fully opaque)
-- Create the image and draw the translucent background
local label = Instance.new("ImageLabel")
label.Size = UDim2.fromScale(1, 1) -- Fill screen
label.BackgroundTransparency = 1
label.Parent = Instance.new("ScreenGui", game.StarterGui)
local image = Instance.new("EditableImage")
image.Size = Vector2.new(1024, 1024) -- Current max size
image.Parent = label
image:DrawRectangle(Vector2.new(), image.Size, Color3.new(), 1 - alpha)
-- Cut the circle (magically)
local radius = 300
local halfSize = image.Size/2
local position = Vector2.new(halfSize.X - radius, halfSize.Y - radius) -- Center
CutCircle(image, position, radius, alpha)