Why Multiscale

One scene, many sizes

Resizing templates is expensive

Flip the resizing onto the image

Scale as a coordinate

A detector uses a $20 \times 20$ pixel bird template and wants to find birds four times larger in size. Why is downsampling the image by a factor of 4 and reusing the template cheaper than stretching the template to $80 \times 80$?

A pyramid shrinks linear size by a factor of $3/4$ per level. A bird template matches at level 0, level 1 and level 3 of the same tree photo. Which birds are the largest in the original image?

A pyramid shrinks linear image size by a factor of $3/4$ at each level. Starting from level 0 at full size, compute the linear scale of level 4 relative to the original, that is $(3/4)^4$, and write it as a two-decimal number.