Package fastjpeg uses djpeg(1), from the Independent JPEG Group's (www.ijg.org) jpeg package, to quickly down-sample images on load. It can sample images by a factor of 1, 2, 4 or 8. This reduces the amount of data that must be decompressed into memory when the full resolution image isn't required, i.e. in the case of generating thumbnails.
DecodeDownsample decodes JPEG data in r, down-sampling it by factor. If djpeg is not found, err is ErrDjpegNotFound and r is not read from. If the execution of djpeg, or decoding the resulting PNM fails, error will be of type DjpegFailedError.
Factor returns the sample factor DecodeSample should use to generate a sampled image greater than or equal to sw x sh pixels given a source image of w x h pixels.
DjpegFailedError wraps errors returned when calling djpeg and handling its response. Used for type asserting and retrying with other jpeg decoders, i.e. the standard library's jpeg.Decode.