Joining operations together

The region create / prepare / prepare / free calls you use to get pixels from an image are an exact parallel to the start / generate / generate / stop calls that images use to create pixels. In fact, they are the same: a region on a partial image holds the state created by that image for the generate function that will fill the region with pixels.

VIPS joins image processing operations together by linking the output of one operation (the start / generate / stop sequence) to the input of the next (the region it uses to get pixels for processing). This link is a single function call, and very fast. Additionally, because of the the split between allocation and processing, once a pipeline of operations has been set up, VIPS is able to run without allocating and freeing memory.

This graph (generated by vipsprofile, the vips profiler) shows memory use over time for a vips pipeline running on a large image. The bottom trace shows total memory, the upper traces show threads calculating useful results (green), threads blocked on synchronisation (red) and memory allocations (white ticks).

Because the intermediate image is just a small region in memory, a pipeline of operations running together needs very little RAM. In fact, intermediates are small enough that they can fit in L2 cache on most machines, so an entire pipeline can run without touching main memory. And finally, because each thread runs a very cheap copy of just the writeable state of the entire pipeline, threads can run with few locks. VIPS needs just four lock operations per output tile, regardless of the pipeline length or complexity.