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README.md

Sixel images in Javascript.

SIXEL image decoding / encoding library for node and the browser.

Decoding

For decoding the library provides a class SixelDecoder with the following properties:

  • constructor(public fillColor: RGBA8888 = DEFAULT_BACKGROUND, public palette: RGBA8888[] = PALETTE_VT340_COLOR, public paletteLimit: number = 65536)
    Creates a new SIXEL image. The optional fillColor (default black) is used to fill "holey pixels" with a background color during pixel transfer in toPixelData. Set palette to the terminal's default colors (defaults to 16 VT340 colors). paletteLimit can be used to restrict the color registers.

  • width: number
    Pixel width of the image. Updates during write. Other than stated in the SIXEL specification (DEC STD 070) a width from raster attributes takes precedence, thus if a SIXEL data stream contains raster attributes with a valid horizontal extend, width will always be set to this value, even for half transmitted images. Also overlong sixel bands (a row of 6 vertical pixels) will be stripped to the raster attribute width. If no raster attributes were transmitted (used in earlier SIXEL variant) width will be set to the longest sixel band width found.

  • height: number
    Pixel height of the image. Updates during write. Height is either set from a valid vertical extend in raster attributes, or grows with the number of sixel bands found in the data stream (number of bands * 6). Raster attributes again have precedence over number of bands (also see width).

  • realWidth: number
    Other than width contains the real width of image data derived from longest SIXEL band found. For spec conform output use this.

  • realHeight: number
    Contains the real height of the image data, which might be greater or lesser than height. For spec conform output use this.

  • rasterWidth: number
    Contains width from raster attributes. 0 if no raster attributes were found.

  • rasterHeight: number
    Contains height from raster attributes. 0 if no raster attributes were found.

  • rasterRatioNumerator: number & rasterRatioDenominator: number
    Contains the pixel ratio numerator and denominator given by raster attributes. Note that toPixelData does not evaluate these settings and always assumes 1:1.

  • fillColor: RGBA8888
    Number respresenting the background fill color. A value of 0 will leave background pixels untouched. The number depends on endianess of the architecture, create it with toRGBA8888(r, g, b, a).

  • memoryUsage: number
    Get current memory usage of the image data in bytes. Can be used to restrict image handling if memory is limited.
    Note: This only accounts the image pixel data storage, the real value will be slightly higher due to some JS object overhead.

  • decode(data: UintTypedArray, start: number = 0, end: number = data.length): void
    Decodes SIXEL bytes and updates the image data. This is done as a stream, therefore it is possible to grab partly transmitted images (see "Simulate slow chunks" in browser example). data can be any array like type with single byte values per index position.
    Note: Normally SIXEL data is embedded in a DCS escape sequence. To properly handle the full sequence with introducer and finalizer you should to use an escape sequence parser (like node-ansiparser or the parser found in xterm.js). The write method of SixelImage is only meant for the data part (also see example node_example_decode_full_sequence.js).

  • decodeString(data: string, start: number = 0, end: number = data.length): void
    Same as write but with string data instead. For better performance use write.

  • toPixelData(target: Uint8ClampedArray, width: number, height: number, dx: number = 0, dy: number = 0, sx: number = 0, sy: number = 0, swidth: number = this.width, sheight: number = this.height, fillColor: RGBA8888 = this.fillColor): Uint8ClampedArray
    Writes pixel data to pixel array target. A pixel array can be obtained from ImageData.data, e.g. from a canvas. width and height must contain the full dimension of the target. Use dx, dy (offset in target) and sx, sy (offset in source) and swidth, sheight (area in source) for cropping/clipping. fillColor has the same meaning as in the constructor, explicit setting it to 0 will leave non encoded pixels unaltered (pixels, that were not colored in the SIXEL data). This can be used for a transparency like effect (background/previous pixel value will remain). Returns the altered target.

Encoding

For encoding the library provides the following properties:

  • sixelEncode(data: Uint8ClampedArray | Uint8Array, width: number, height: number, palette: RGBA8888[] | RGBColor[], rasterAttributes: boolean = true): string
    Encodes pixel data to a SIXEL string. data should be an array like type with RGBA pixel data. width and height must contain the pixel dimension of data. palette should contain the used colors in data and must not be empty. To avoid poor output quality consider using a quantizer with dithering and palette creation before converting to SIXEL. See node_example_encode.js for an example usage in conjunction with rgbquant. For transparency only an alpha value of 0 will be respected as fully transparent, other alpha values are set to fully opaque (255). Transparent pixels will be colored by the terminal later on depending on the backgroundSelect setting of the introducer.
    Note: Some terminals have strict palette limitations, in general the palette should not contain more than 256 colors.

  • introducer(backgroundSelect: number = 0): string
    Creates the escape sequence introducer for a SIXEL data stream. This should be written to the terminal before any SIXEL data.
    backgroundSelect is a hint for the terminal how to deal with uncolored pixels:

    • 0 - device default action (most terminals will apply background color)
    • 1 - no action (previous pixel value at output position should remain)
    • 2 - set to background color (device dependent)
  • FINALIZER: string
    Finalizes the SIXEL escape sequence. Write this, when the SIXEL data stream has ended. Note that a SIXEL escape sequences changes the operation mode of a terminal, forgetting the finalizer might leave the terminal in an unrecoverable state.

Convenient Properties

Furthermore the library exposes some convenient properties:

  • function toRGBA8888(r: number, g: number, b: number, a: number = 255): RGBA8888
    Converts the RGBA channel values to the native color type RGBA8888.

  • function fromRGBA8888(color: RGBA8888): number[]
    Converts the native color to an array of [r, g, b, a].

  • PALETTE_VT340_COLOR: RGBA8888[]
    16 color palette of VT340.

  • PALETTE_VT340_GREY: RGBA8888[]
    16 monochrome palette of VT340.

  • PALETTE_ANSI_256: RGBA8888[]
    256 ANSI color palette derived from xterm.

Installation

Install the library with npm install sixel.

Examples and browser demo

See the example files for decoding/encoding in nodejs. Note that the examples and the browser demo are not part of the npm package, clone the repo and run npm install if you want to see them in action.

Decoding can also be tested in the browser after npm start under localhost:8080.

Encoding can be tested in a SIXEL capable terminal with img2sixel.js, e.g.

$> node img2sixel.js -p16 http://leeoniya.github.io/RgbQuant.js/demo/img/bluff.jpg

Benchmarks

Performance is measured for typical actions based on 9-bit palette image: test image

The test image repeats the palette image 6 times to form a 640x480 image with 512 colors. The unusual (and not spec conform) high number of colors was chosen to explicit test for this as an upper bound.

Results:

   Context "lib/index.benchmark.js"
      Context "testimage"
         Context "pixel transfer"
            Case "toPixelData - with fillColor" : 10 runs - average runtime: 1.86 ms
            Case "toPixelData - without fillColor" : 10 runs - average runtime: 1.28 ms
         Context "decode"
            Case "decode" : 10 runs - average runtime: 4.22 ms
            Case "decodeString" : 10 runs - average runtime: 6.60 ms
            Case "decode + pixel transfer" : 10 runs - average runtime: 4.33 ms
         Context "encode"
            Case "sixelEncode" : 10 runs - average runtime: 25.92 ms
      Context "decode - testfiles"
         Case "test1_clean.sixel" : 10 runs - average runtime: 17.96 ms
         Case "test2_clean.sixel" : 10 runs - average runtime: 8.01 ms
         Case "sampsa1_clean.sixel" : 10 runs - average runtime: 43.30 ms

Note that encoding is much more expensive than decoding and prolly should be called within a webworker or child process.

Status

Currently beta, still more tests to come.

References

While being quite common in the DEC ecosystem in the 80s (even used for printer protocols), SIXEL references are very limited these days. The closest to a specification we have can be found in the Video Systems Reference Manual (DEC STD 070, p. 908-933). Also see Sixel Graphics on vt100.net, which gives a quick overview. For implementation the old usenet article "All About SIXELs" was very helpful.

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