Does libjpeg allows/have routines to scale the output image?
I have an image which needs to be resized when saving to and loading from, Providing width1, height1 on input and getting width2, height2 on output, and I really cant use anything outside.
Yes, libjpeg does support image scaling, but with some limitations.
Libjpeg version 8c and higher supports M/8 scaling where M is an integer between 1 and 16. The more commonly used libjpeg version 6b supports M of 1, 2, 4, and 8 only (i.e. power-of-2 downscaling).
Supply the scaling numerator and denominator to the decompression object using scale_num and scale_denom. See jpeg_decompress_struct in jpeglib.h. You can refer to djpeg.c for a usage example.
This is much faster, and potentially higher quality, than decoding the whole image and then downscaling.
No, you need to scale it either by yourself (linear interpolation is the easiest and fastest but gives square shaped pixels, bilinear/trilinear/bicubic are trickiers but give smoother results), or by using a library that will do the dirty work for you (check OpenCV, SDL, imagemagick, libpipi, etc.)
Actual code for a simple nearest neighbor interpolation (unoptimized) :
dest[dx+dy*dest_width] = src[(dx*src_width/dest_width)+(dy*src_height/dest_height)*src_width]
Just iterate dx and dy according to the dest_width and dest_height. But really, use a library for this.
来源：https://stackoverflow.com/questions/7488048/libjpeg-output-scaling