png-dpi-reader

png画像のpHYs chunkを読んでdpiを算出する

code:mod.ts

import {Buffer} from 'deno'

import {isPng, toDec, readIHDR, getCharCodes, ImageInfo} from 'https://scrapbox.io/api/code/daiiz-deno/png-dpi-reader-writer/share.ts'

export async function parsePngFormat (buf: Buffer): Promise<ImageInfo> {

return readChunks(buf)

}

code:mod.ts

const readpHYs = async (buf: Buffer): Promise<number> => {

let p

// https://tools.ietf.org/html/rfc2083#page-22

p = new Uint8Array(4); await buf.read(p)

const pixelsPerUnitXAxis = toDec(p)

p = new Uint8Array(4); await buf.read(p)

const pixelsPerUnitYAxis = toDec(p)

p = new Uint8Array(1); await buf.read(p)

const unitSpecifier = p0

let dpi = 72

if (unitSpecifier === 1) {

// calculate dots per inch

dpi = Math.floor(Math.max(pixelsPerUnitXAxis, pixelsPerUnitYAxis) / (unitSpecifier * 39.3))

}

return dpi

}

const readChunks = async (buf: Buffer): Promise<ImageInfo> => {

let info: ImageInfo = {}

if (!await isPng(buf)) return info

const {width, height} = await readIHDR(buf)

info.width = width

info.height = height

while (true) {

if (buf.empty()) break

let p

p = new Uint8Array(4); await buf.read(p)

const chunkLength = toDec(p)

p = new Uint8Array(4); await buf.read(p)

const chunkType = p.join(' ')

if (chunkType === getCharCodes('IDAT') || chunkType === getCharCodes('IEND')) break

switch (chunkType) {

case getCharCodes('pHYs'):

info.dpi = await readpHYs(buf)

break

default:

p = new Uint8Array(chunkLength); await buf.read(p)

}

// CRC

p = new Uint8Array(4); await buf.read(p)

}

return info

}