nimPNG (PNG + APNG)

Portable Network Graphics Encoder and Decoder written in Nim store lossless image with good compression. Since version 0.2.0 also support Animated PNG!

all PNG standard color mode are supported:

• LCT_GREY = 0, # greyscale: 1,2,4,8,16 bit
• LCT_RGB = 2, # RGB: 8,16 bit
• LCT_PALETTE = 3, # palette: 1,2,4,8 bit
• LCT_GREY_ALPHA = 4, # greyscale with alpha: 8,16 bit
• LCT_RGBA = 6 # RGB with alpha: 8,16 bit

both interlaced and non-interlaced mode supported

recognize all PNG standard chunks: IHDR, IEND, PLTE, IDAT, tRNS, bKGD, pHYs, tIME, iTXt, zTXt tEXt, gAMA, cHRM, sRGB, iCCP, sBIT, sPLT, hIST

unknown chunks will be handled properly

the following chunks are supported (generated/interpreted) by both encoder and decoder:

• IHDR: header information
• PLTE: color palette
• IDAT: pixel data
• IEND: the final chunk
• tRNS: transparency for palettized images
• tEXt: textual information
• zTXt: compressed textual information
• iTXt: international textual information
• bKGD: suggested background color
• pHYs: physical dimensions
• tIME: modification time

the following chunks are parsed correctly, but not used by decoder: cHRM, gAMA, iCCP, sRGB, sBIT, hIST, sPLT

Supported color conversions:

• anything to 8-bit RGB, 8-bit RGBA, 16-bit RGB, 16-bit RGBA
• any grey or grey+alpha, to grey or grey+alpha
• anything to a palette, as long as the palette has the requested colors in it
• removing alpha channel
• higher to smaller bitdepth, and vice versa

Planned Feature(s):

• streaming for progressive loading

Basic Usage

import nimPNG

let png = loadPNG32("image.png")
#is equivalent to:
#let png = loadPNG("image.png", LCT_RGBA, 8)
#will produce rgba pixels:
#png.width -> width of the image
#png.height -> height of the image
#png.data -> pixels data in RGBA format

if you already have the whole file in memory:

let png = decodePNG32(raw_bytes)
#will do the same as above

other variants:

• loadPNG24 -> will produce pixels in RGB format 8 bpp
• decodePNG24 -> load png from memory instead of file

to create PNG:

• savePNG32("output.png", rgba_pixels, width, height) or savePNG24
• encodePNG32(rgba_pixels, width, height) or encodePNG24

special notes:

• Use loadPNG or savePNG if you need specific input/output format by supplying supported colorType and bitDepth information.
• Use encodePNG or decodePNG to do in-memory encoding/decoding by supplying desired colorType and bitDepth information

pixels are stored as raw bytes using Nim's string as container:

Byte Order	Format
r1,g1,b1,a1,...,rn,gn,bn,an	RGBA 8 bit
r1,g1,b1,r2,g2,b2,...,rn,gn,bn	RGB 8 bit
grey1,grey2,grey3, ..., greyn	GREY 8 bit
grey1,a1,grey2,a2,...,greyn,an	GREY ALPHA 8 bit

Animated PNG (APNG)

Since version 0.2.0, nimPNG provides support for Animated PNG.

Both decoder and encoder recognize/generate APNG chunks correctly: acTL, fcTL, fdAT.

Decoded frames is provided as is, the dimension and coordinate offset might be different with default frame. No alpha blending or other blending method performed. It is up to the application to do proper in-memory rendering before displaying the animation. Don't ask how to do it, any decent graphics rendering library have their own set of API to do alpha blending and offset rendering. In the future nimPNG might be shipped with simple frame rendering utility for common cases. Right now nimPNG is just a PNG encoder/decoder.

Decoding

#let png = loadPNG32("image.png")
# or
#let png = loadPNG("image.png", LCT_RGBA, 8)
# or
#let png = decodePNG32(raw_bytes)

The usual loadPNG and decodePNG can decode both unanimated and animated PNG. png.width, png.height, png.data works as usual. If the decoded PNG is an APNG, png.data will contains default frame. Animation frames can be accessible via png.frames. If it is not an APNG, png.frames will be nil.

Encoding

var png = prepareAPNG24(numPlays)

• First step is to call prepareAPNG, prepareAPNG24, or prepareAPNG32. You also can specify how many times the animation will be played

  png.addDefaultImage(framePixels, w, h, ctl)

• Second step is also mandatory, you should call addDefaultImage. ctl is optional, if you provide a ctl(Frame Control), the default image will be part of the animation. If ctl is nil, default image will not be part of animation.

  png.addFrame(frames[i].data, ctl)

• Third step is calling addFrame one or more times. Here ctl is mandatory.

  png.saveAPNG("rainbow.png")
  # or
  var str = png.encodeAPNG()

• Final step is to call saveAPNG if you want save it to file or call encodeAPNG if you want to get the result in a string container

You can read the details of frame control from spec. You can also see an example in tester/test.nim -> generateAPNG