Method

VipsImageunpremultiply

Declaration [src]

int
vips_unpremultiply (
  VipsImage* in,
  VipsImage** out,
  ...
)

Description [src]

Optional arguments:

max_alpha: %gdouble, maximum value for alpha
alpha_band: %gint, band containing alpha data

Unpremultiplies any alpha channel. Band alpha_band (by default the final band) contains the alpha and all other bands are transformed as:

  alpha = (int) clip(0, in[in.bands - 1], @max_alpha);
  norm = (double) alpha / @max_alpha;
  if (alpha == 0)
    out = [0, ..., 0, alpha];
  else
    out = [in[0] / norm, ..., in[in.bands - 1] / norm, alpha];

So for an N-band image, the first N - 1 bands are divided by the clipped and normalised final band, the final band is clipped. If there is only a single band, the image is passed through unaltered.

The result is

VIPS_FORMAT_FLOAT unless the input format is #VIPS_FORMAT_DOUBLE, in which

case the output is double as well.

max_alpha has the default value 255, or 65535 for images tagged as

VIPS_INTERPRETATION_RGB16 or

VIPS_INTERPRETATION_GREY16.

Non-complex images only.

See also: vips_premultiply(), vips_flatten().

This method is not directly available to language bindings.

Parameters

out

Type: VipsImage

Output image.

The argument will be set by the function.

The caller of the method takes ownership of the returned data, and is responsible for freeing it.

...

Type:

NULL-terminated list of optional named arguments.

Return value

Type: int

0 on success, -1 on error.