Resize image multiple times

I had a look through the docs/wiki and couldn’t see any examples of what I’m trying to do, which is save multiple sizes of the same image after cropping:

sharp( imgBuffer )
    .resize(800, 800)
    .crop(sharp.gravity.centre)
    .toFile( imgLarge )
    .resize(300, 300)
    .toFile( imgMedium )
    .resize(100, 100)
    .toFile( imgSmall );

Would something like that work?

[lovell]

Hi Jonathan,

An undocumented feature is that if the input is a filename, you can do exactly what you’re trying to achieve.

You’re using a Buffer, which behaves slightly differently to work around a nuance of the V8 garbage collector.

An input Buffer containing compressed image data is copied and unreferenced as soon as the first call to toFile (or toBuffer or pipe) is made. This is to avoid the situation where, should the memory compaction stage of the V8 garbage collector run in the time between a resize task being queued and being processed, the Buffer could move without us knowing.

In your example this means the internal reference to imgBuffer held by the instance of sharp doesn’t exist by the second call to toFile.

To provide the API you’re expecting, we’d need to change this module to hold two copies of the input Buffer in memory at the same time, one in JavaScript land and one in C++ land. The newer version of V8 and the changes to Buffer allocation in Node.js 0.11 unstable (and io.js) might improve the situation, so I’d be happy to revisit this when they stabilise.

As an aside, the default behaviour is to crop with centre gravity so you can leave the .crop(sharp.gravity.centre) bit out.

[nmec]

Not in a single chain like I outlined above, instead I keep the image buffer in memory and loop over it for each size I need:

var sizes = [{
    path: 'large',
    xy: 800
},{
    path: 'medium',
    xy: 300
},{
    path: 'small',
    xy: 100
}];

Promise.map(sizes, function(size) {
    return sharp( img.buffer )
        .resize( size.xy, size.xy )
        .toFile( size.path );
});