sharp/docs/api-channel.md

removeAlpha

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

See also flatten.

Examples

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

Returns Sharp

ensureAlpha

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.

Parameters

• alpha number alpha transparency level (0=fully-transparent, 1=fully-opaque) (optional, default 1)

Examples

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

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

• Throws Error Invalid alpha transparency level

Returns Sharp

Meta

• since: 0.21.2

extractChannel

Extract a single channel from a multi-channel image.

Parameters

• channel (number | string) zero-indexed channel/band number to extract, or red, green, blue or alpha.

Examples

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

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

• Throws Error Invalid channel

Returns Sharp

joinChannel

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.

Parameters

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

• Throws Error Invalid parameters

Returns Sharp

bandbool

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

Parameters

• boolOp string one of and, or or eor to perform that bitwise operation, like the C logic operators &, | and ^ respectively.

Examples

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`.
  });

• Throws Error Invalid parameters

Returns Sharp