= Depth map
A <depth map> assigns a distance-related value to each accepted image location. The geometry must specify whether this means depth along the <camera> axis or <Euclidean distance> along the viewing ray. Units, missing values and the coordinate frame are also part of the representation. In calibrated <stereoscopy>, converting a <disparity map> through the camera geometry produces depth estimates.<Depth Map from Stereo Images in the OpenCV-Python tutorials>[Basics, disparity inversely proportional to depth]{ref}
For an ideal camera, suppose a point has coordinates $(X,Y,Z)=(1,0,2)$ metres. Its axial depth is two metres, but its <Euclidean distance> from the <camera center> is $\sqrt5$ metres. The same scene can thus yield different valid numerical maps under different conventions. A missing <stereo correspondence> requires an invalid-location marker rather than a fabricated depth. <Camera calibration> and <measurement uncertainty> determine how confidently accepted values describe the scene.
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