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