Playing cards tend to be a particular size, for example. The CN tower is a particular size, we may know. We can see parts that are usually concealed. The illusion is said to applied correctly in a full picture. Likely several factors contribute to particular illusions.
Shape constancy is achieved despite variations in orientation.
If indicators of slant are present they influence apparent shape. Also, the shape of a familiar object can influence apparent orientation. Lightness constancy involves reporting the reflectance or albedo of a surface. Brightness is a term used to refer to the apparent amount of light entering the eye, and intensity to the actual amount. The Retinex theory contends we use the ratio principle at sharp contours.