Aperture/mask for multiple images

Online experiments
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4

Try the following code in Begin Experiment

vertices = []
ovalY = .15 # vertical radius
ovalX = .2 # horizontal radius
rectY = .2 # vertical outer boundary
rectX = .4 # horizontal outer boundary
nPoints = 72 # oval resolution
for Idx in range(nPoints+1):
     vertices.append([ovalX*cos(2*pi*Idx/nPoints),ovalY*sin(2*pi*Idx/nPoints)])
vertices.append([rectX,rectY])
vertices.append([rectX,-rectY])
vertices.append([-rectX,-rectY])
vertices.append([-rectX,rectY])
vertices.append([rectX,rectY])
vertices.append([ovalX,0])

Then set up a custom polygon with vertices equal to vertices (or create a polygon in code) in the same colour as your background

Alternatively