Image Pyramid
Also known as: mipmap pyramid, multi resolution image, image pyramid, multiresolution pyramid
An image pyramid is a set of progressively smaller copies of the same image, each at half the resolution of the one below. Apps pick the level closest to the size they need, which speeds up rendering and is the idea behind mipmaps and gallery thumbnails.
- Each pyramid level is roughly half the resolution of the one below, from full size up to a tiny preview.
- Mipmaps in graphics and gallery thumbnails are both practical image pyramids.
- Cached pyramid levels speed up browsing but add to thumbnail cache and can be safely regenerated.
What an image pyramid is
An image pyramid stores one picture at multiple resolutions, stacked from full size at the base to a tiny version at the top, with each level typically halved in width and height. A Gaussian pyramid smooths and downsamples to build these levels; a Laplacian pyramid stores the differences between levels and is used in blending and compression. In real-time graphics the same concept is called a mipmap, where pre-computed smaller textures are selected based on how far away a surface is.
The point is to avoid resampling a huge image every time you need a small version. Instead of scaling a multi-megapixel photo down on the fly for a list cell, an app reads the pyramid level that already matches the target size. That is faster, uses less memory, and produces cleaner results because the downscaled levels were generated with proper filtering.
Pyramids, thumbnails, and storage on phones
Photo galleries lean on this idea constantly. A grid of hundreds of photos shows small previews, while tapping one shows a larger render. Rather than decoding each original at full resolution, the gallery keeps cached thumbnails at a few sizes, effectively a pyramid, in a thumbnail cache. On iOS this is managed through the photo library and PhotoKit; on Android image loaders and MediaStore thumbnails play a similar role.
Those cached levels are convenient but they accumulate, which is part of how galleries and other apps quietly grow their app cache and system data. A cleaner like Cleanor helps by clearing regenerable thumbnail caches to reclaim space, while the original full-resolution photos, the base of the pyramid, stay untouched. The cached small versions simply rebuild on demand the next time you browse.