sixel.go

package sixel

import (
	"bufio"
	"bytes"
	"errors"
	"fmt"
	"image"
	"image/color"
	"image/draw"
	"io"
	"os"

	"github.com/soniakeys/quant/median"
)

// Encoder encode image to sixel format
type Encoder struct {
	w io.Writer

	// Dither, if true, will dither the image when generating a paletted version
	// using the Floyd–Steinberg dithering algorithm.
	Dither bool

	// Width is the maximum width to draw to.
	Width int
	// Height is the maximum height to draw to.
	Height int

	// Colors sets the number of colors for the encoder to quantize if needed.
	// If the value is below 2 (e.g. the zero value), then 255 is used.
	// A color is always reserved for alpha, so 2 colors give you 1 color.
	Colors int
}

// NewEncoder return new instance of Encoder
func NewEncoder(w io.Writer) *Encoder {
	return &Encoder{w: w}
}

const (
	specialChNr = byte(0x6d)
	specialChCr = byte(0x64)
)

// Encode do encoding
func (e *Encoder) Encode(img image.Image) error {
	nc := e.Colors // (>= 2, 8bit, index 0 is reserved for transparent key color)
	if nc < 2 {
		nc = 255
	}

	width, height := img.Bounds().Dx(), img.Bounds().Dy()
	if width == 0 || height == 0 {
		return nil
	}
	if e.Width > 0 {
		width = e.Width
	}
	if e.Height > 0 {
		height = e.Height
	}

	var paletted *image.Paletted

	// fast path for paletted images
	if p, ok := img.(*image.Paletted); ok && len(p.Palette) < int(nc) {
		paletted = p
	} else {
		// make adaptive palette using median cut alogrithm