Method

VipsImagepremultiply

Declaration [src]

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

Description [src]

Premultiplies any alpha channel.

The final band is taken to be the alpha and the bands are transformed as:

alpha = clip(0, in[in.bands - 1], max_alpha)
norm = alpha / max_alpha
out = [in[0] * norm, ..., in[in.bands - 1] * norm, alpha]

So for an N-band image, the first N - 1 bands are multiplied 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 VipsBandFormat unless the input format is VipsBandFormat, in which case the output is double as well.

max_alpha has the default value 255, or 65535 for images tagged as VipsInterpretation or VipsInterpretation, and 1.0 for images tagged as VipsInterpretation.

Non-complex images only.

::: tip “Optional arguments” * max_alpha: gdouble, maximum value for alpha

::: seealso vips_unpremultiply(), vips_flatten().

Parameters

`out`	`VipsImage`
	Output image.
	The argument will be set by the function.
	The instance takes ownership of the data, and is responsible for freeing it.
`...`	`none`
	`NULL`-terminated list of optional named arguments.

Return value

Returns:	`int`
	0 on success, -1 on error.