Enhancement: allow ensureAlpha to set the alpha transparency level

[mtaylor-tbpm]

I'm re-writing an older .NET program that uses ImageMagick and need help mapping image operations. I'm also not very familiar with image processing so bear with me :) Using Node.js and the npm Sharp package.

The program has multiple conversion paths/outputs per input image and Sharp is working great except for one path where the ImageMagick output looks different from the Sharp output. The IM image is fully transparent and the Sharp image has visible areas. I suspect it's related to alpha and transparency.

Below is a sample of the old code. I'm not sure about the FilterType (kernel?) and the transparent option. I also attached examples of an input image and outputs from IM and Sharp. Any help you can provide would be great. Thank you!

MagickImage image = new MagickImage('input.png');
image.FilterType = FilterType.Point;
image.Scale(2500, 2500);
image.Alpha(AlphaOption.Transparent);
image.ColorType = ColorType.TrueColorAlpha;
image.Format = MagickFormat.Png64;
…process/save image

I've gotten this far...

sharp(“test.png”)
.resize({
height: 2500,
width: 2500,
kernel: ?,
})
.toColorspace('rgb16')
.toFile(“output.png”)

Input

ImageMagick output

Sharp output

[lovell]

Hello, the Point filter in ImageMagick uses nearest-neighbour interpolation so try the 'nearest' kernel in sharp for its equivalent.

The AlphaOption.Transparent operation in ImageMagick creates and/or sets a fully-transparent alpha channel. The closest feature in sharp is ensureAlpha, equivalent to AlphaOption.Opaque.

This doesn't quite do what you need, but it would be a nice addition to sharp for ensureAlpha to optionally set a transparency level between 0 (default, fully-opaque) and 1 (fully-transparent).

Here's how the API to do this might look:

// PROPOSED API, NOT YET AVAILABLE

sharp(input).ensureAlpha(1)... // fully-transparent

[lovell]

v0.28.1 allows the alpha transparency level to be set via ensureAlpha.

https://sharp.pixelplumbing.com/api-channel#ensurealpha

[mattpr]

The docs say:

This is a no-op if the image already has an alpha channel.

So ensureAlpha will only set alpha if there isn't already an alpha channel present?
Is there another way to change the existing alpha/transparency?
Use case is when building layers for compositing and needing to adjust transparency of the entire image.

[lovell]

@mattpr You can chain .removeAlpha().ensureAlpha(requiredAlpha) in that order.