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

[lovell]

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.

[Lanfei]

Is the before function not been supported yet?

[lovell]

@Lanfei Not yet, sorry, but please do subscribe to this issue for updates.

[Lanfei]

Okay.

[jaredscheib]

HI @lovell, I think this Issue addresses what I'm trying to solve currently as well – I've subscribed for notifications for when .before is implemented.

In the meantime, might you have a suggestion for how I can verify that a stream contains valid image data before piping it into a transform? I'm using request to pipe data from various URLs that may or may not point at valid images.

I tried piping into a sharp().metadata() instance, handling any errors, and then continue to pipe the stream from inside the success callback (which would seem to imply that it successfully read metadata from a valid image), but I get the error: You cannot pipe after data has been emitted from the response. I understand why this is happening but haven't come up with a solution to pause the stream and then resume from inside the callback, for instance.

[lovell]

@jaredscheib This feature (possibly with #298) should allow you to achieve what you need, yes. In the meantime, the least complex (and race-condition free) method of doing so is probably to store the streamed data in a Buffer.