Channel manipulation

removeAlpha

removeAlpha() ⇒ Sharp

Remove alpha channels, if any. This is a no-op if the image does not have an alpha channel.

See also flatten.

Example

sharp('rgba.png')
  .removeAlpha()
  .toFile('rgb.png', function(err, info) {
    // rgb.png is a 3 channel image without an alpha channel
  });

ensureAlpha

ensureAlpha([alpha]) ⇒ Sharp

Ensure the output image has an alpha transparency channel. If missing, the added alpha channel will have the specified transparency level, defaulting to fully-opaque (1). This is a no-op if the image already has an alpha channel.

Throws:

• Error Invalid alpha transparency level

Since: 0.21.2

Param	Type	Default	Description
[alpha]	number	1	alpha transparency level (0=fully-transparent, 1=fully-opaque)

Example

// rgba.png will be a 4 channel image with a fully-opaque alpha channel
await sharp('rgb.jpg')
  .ensureAlpha()
  .toFile('rgba.png')

Example

// rgba is a 4 channel image with a fully-transparent alpha channel
const rgba = await sharp(rgb)
  .ensureAlpha(0)
  .toBuffer();

extractChannel

extractChannel(channel) ⇒ Sharp

Extract a single channel from a multi-channel image.

The output colourspace will be either b-w (8-bit) or grey16 (16-bit).

Throws:

• Error Invalid channel

Param	Type	Description
channel	number | string	zero-indexed channel/band number to extract, or red, green, blue or alpha.

Example

// green.jpg is a greyscale image containing the green channel of the input
await sharp(input)
  .extractChannel('green')
  .toFile('green.jpg');

Example

// red1 is the red value of the first pixel, red2 the second pixel etc.
const [red1, red2, ...] = await sharp(input)
  .extractChannel(0)
  .raw()
  .toBuffer();

joinChannel

joinChannel(images, options) ⇒ Sharp

Join one or more channels to the image. The meaning of the added channels depends on the output colourspace, set with toColourspace(). By default the output image will be web-friendly sRGB, with additional channels interpreted as alpha channels. Channel ordering follows vips convention:

• sRGB: 0: Red, 1: Green, 2: Blue, 3: Alpha.
• CMYK: 0: Magenta, 1: Cyan, 2: Yellow, 3: Black, 4: Alpha.

Buffers may be any of the image formats supported by sharp. For raw pixel input, the options object should contain a raw attribute, which follows the format of the attribute of the same name in the sharp() constructor.

Throws:

• Error Invalid parameters

Param	Type	Description
images	Array.<(string|Buffer)> | string | Buffer	one or more images (file paths, Buffers).
options	Object	image options, see sharp() constructor.

bandbool

bandbool(boolOp) ⇒ Sharp

Perform a bitwise boolean operation on all input image channels (bands) to produce a single channel output image.

Throws:

• Error Invalid parameters

Param	Type	Description
boolOp	string	one of and, or or eor to perform that bitwise operation, like the C logic operators &, `

Example

sharp('3-channel-rgb-input.png')
  .bandbool(sharp.bool.and)
  .toFile('1-channel-output.png', function (err, info) {
    // The output will be a single channel image where each pixel `P = R & G & B`.
    // If `I(1,1) = [247, 170, 14] = [0b11110111, 0b10101010, 0b00001111]`
    // then `O(1,1) = 0b11110111 & 0b10101010 & 0b00001111 = 0b00000010 = 2`.
  });