nimPNG

Portable Network Graphics Encoder and Decoder written in Nim store lossless image with good compression

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.

Decoding

#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