greyscale().raw() sets wrong OutputInfo.channels

[hallsbyra]

After doing a grayscale() operation, OutputInfo.channels = 3, when it really should be 1.

Example:

const logo = await logoSrc
    .greyscale()
    .raw()
    .toBuffer({ resolveWithObject: true })
// This is set to 3, which is wrong. It should be 1 since the picture is grayscale. Have to manually adjust it to 1.
logo.info.channels = 1

[lovell]

Hello, did you see the greyscale docs?

"By default the output image will be web-friendly sRGB and contain three (identical) color channels. This may be overridden by other sharp operations such as toColourspace('b-w'), which will produce an output image containing one color channel."

[hallsbyra]

Yes, but I'm not sure it's correct. Look at this:

const gray = await sharp({create: {width: 100, height: 100, channels: 3, background: {r: 255, g: 255, b: 255}}})
    .greyscale()
    .raw()
    .toBuffer({resolveWithObject: true})
// Comment out this line to get: VipsImage: memory area too small --- should be 30000 bytes, you passed 10000
gray.info.channels = 1
const out = await sharp(gray.data, {raw: {width: gray.info.width, height: gray.info.height, channels: gray.info.channels}})
    .raw()
    .toBuffer()

After the first operation, gray.info.channels = 3. Ok. As per the docs. Problem is, gray.data.length = 10000. And that's just one channel (100x100), or what am I getting wrong? Forcing it to one channel makes the second operaton work.

[lovell]

Yes, sorry, using raw output extracts a single channel when greyscale is also used so this is a bug.

Happy to accept and/or help with a PR if you're able to add the fix and a test case. baton->channels needs to be modified inside the if block approximately here:

sharp/src/pipeline.cc

Lines 771 to 774 in de11d36

	if (baton->greyscale || image.interpretation() == VIPS_INTERPRETATION_B_W) {
	// Extract first band for greyscale image
	image = image[0];
	}

[lovell]

Commit 6f9699f adds a test assertion that would have caught this, plus fixes the bug. This will be in v0.21.1, thanks for reporting.

[lovell]

v0.21.1 is now available with this fix.