Improvements to the handling of images with multiple alpha channels

[webbery]

Hi, I use sharp v0.25.4. I load a 16bit tiff and want to display on web canvas. Web chanvas require 4 channel containning alpha. In my code, I write it like this:

let {data, info} = await sharp(tiffpath).jpeg({force: true}).ensureAlpha()
          .raw().toBuffer({ resolveWithObject: true })

I see info is still 6 channel but not 4. Then my code throw an error which can not create ImageData. Is it a bug?

[lovell]

Hi, are you able to provide a sample input image?

As an aside, the use of jpeg() in this code sample is ignored as the use of raw() later in the chain takes precedence.

[webbery]

Here is my image. sample.zip Although I can convert 6 channel to 4 channel, I think it's slowly in javscript. It is best to finish this work in C++.

[lovell]

This input TIFF image has 5 channels, RGB and what appears to be two alpha channels, which is unusual.

The raw, uncompressed output pixel data has five channels, for example:

const { info } = await sharp('Shipyard Cranes Masked (8).tif')
  .raw()
  .toBuffer();

produces:

{
  format: 'raw',
  width: 3363,
  height: 4000,
  channels: 5,
  premultiplied: false,
  size: 67260000
}

sharp currently doesn't understand "5 channel RGB" and does not know how treat channels 4 and 5, so using ensureAlpha() with this image adds a 6th channel.

const { info } = await sharp('Shipyard Cranes Masked (8).tif')
  .ensureAlpha()
  .raw()
  .toBuffer();

produces:

{
  format: 'raw',
  width: 3363,
  height: 4000,
  channels: 6,
  premultiplied: false,
  size: 80712000
}

There are two possible improvements here.

1. ensureAlpha (and removeAlpha) might be able to better detect multiple alpha channels.
2. raw should ignore/remove secondary alpha channels to meet its documented behaviour of providing RGBA.

[JamieVangeysel]

I don't use ensureAlpha or removeAlpha. For me it would work if the extra alpha channels are just ignored. The easiest way to implement that in code is by adding an option to the constructor ignoreSecondaryAlpha or something like that defaulting to false to not break alrady existing code