Provide pre-pipeline function to modify options based on input image metadata

This could seriously increase the power of Stream-based image processing.

Here’s an example of how halving an image’s dimensions could work:

// PROPOSED API NOT YET AVAILABLE
var halver = sharp().before(function(metadata) {
  this.resize(metadata.width / 2, metadata.height / 2);
});
readableStream.pipe(halver).pipe(writableStream);
// PROPOSED API NOT YET AVAILABLE

The existing metadata logic can also be improved to require only the first few hundred bytes of a Stream.

In addition, adding a “playbook” of example uses for this feature to the docs would be great too. There’s a variable/percentage extract/crop under discussion at #205 that this should allow for.

[gwen1230]

Any news for this feature ?

[teoxoy]

It will be nice to have this feature but I’m using image-size-stream as an alternative for now.

Example:

const fileStream = fs.createReadStream(path)
const sizeStream = new ImageDimensionsStream()
const bufferStream = new PassThrough().pause()

let stream = fileStream.pipe(sizeStream).pipe(bufferStream)

sizeStream.on('dimensions', ({ width, height }) => {
    // take decision based on image size
    if (W !== 0 && H !== 0 && (X !== 0 || Y !== 0 || width !== W || height !== H)) {
        const extractStream = sharp()
            .extract({ left: X, top: Y, width: W, height: H })
        stream = stream.pipe(extractStream)
    }
    bufferStream.resume()
    res.send(stream)
})

Using a paused PassThrough stream as buffer while ImageDimensionsStream returns the image dimensions works out nicely.