# README

gomp

Go library for image blending and alpha compositing using advanced features like the Porter-Duff operator and blending modes.

About

The reason why this package has been developed is because the image/draw package from Go's standard library defines only one operation: drawing a source image onto the destination image, through an optional image mask. This is performed pixel by pixel and it's based on the classic "Compositing Digital Images" paper by Porter and Duff. This paper presented 12 different composition operation, but the Draw method uses only two of them: source over destination and source.

When dealing with image composition this is simply not enough. This library aims to overcome this deficiency by integrating the missing operators.

Alpha compositing

Blending modes

For convenience, this package implements also some of the most used blending modes in Photoshop. Similarly to the alpha compositing, blending modes defines the result of compositing a source and a destination but without being constrained to the alpha channel. The implementation follows the blending formulas presented in the W3C document: Compositing and Blending. These blending modes are not covered by Porter and Duff, but have been included into this package for convenience.

Blending modes

Installation

$ go install github.com/esimov/gomp@latest

API

The API of the library is inspired by the PorterDuff.Mode class from the Android SDK.

Alpha compositing

op := gomp.InitOp()
op.Set(gomp.SrcOver)
bmp := gomp.NewBitmap(image.Rect(0, 0, size, size))
imop.Draw(bmp, source, backdrop, nil)

Blending modes

op := gomp.NewBlend()
op.Set(gomp.Multiply)
bmp := gomp.NewBitmap(image.Rect(0, 0, size, size))
imop.Draw(bmp, source, backdrop, op)

You can combine the alpha compositing with blending modes at the same step, you just need to replace the last parameter of the Draw method with the initialized blending operation.

Alpha compositing and blending modes combined

imop := gomp.InitOp()
blop := gomp.NewBlend()
blop.Set(gomp.Multiply)
imop.Set(gomp.SrcOver)
bmp := gomp.NewBitmap(image.Rect(0, 0, size, size))
imop.Draw(bmp, srcImg, bgr, blop)

Operators

Image compositing	Separable blending modes	Non-separable blending modes
`Clear`	`Normal`	`Hue`
`Copy`	`Darken`	`Saturation`
`Dst`	`Lighten`	`ColorMode`
`SrcOver`	`Multiply`	`Luminosity`
`DstOver`	`Screen`
`SrcIn`	`Overlay`
`DstIn`	`SoftLight`
`SrcOut`	`HardLight`
`DstOut`	`ColorDodge`
`SrcAtop`	`ColorBurn`
`DstAtop`	`Difference`
	`Exclusion`

Examples

The images used in this document for visualizing the alpha compositing operation and the blending modes have been generated using this library. They can be found in the examples folder.