png-dpi-writer

png画像のpHYs chunkにdpiを書き込む

png-dpi-reader-writerの一部

code:mod.ts

import {Buffer} from 'deno'

import {crc} from 'https://scrapbox.io/api/code/daiiz-deno/crc32/mod.ts'

import {parsePngFormat} from 'https://scrapbox.io/api/code/daiiz-deno/png-dpi-reader/mod.ts'

export async function writePngDpi (src: Buffer, dpi: number): Promise<Uint8Array> {

const insertPosition = await getInsertPosition(src)

if (insertPosition < 0) return src.bytes()

const imgHeader = new Uint8Array(insertPosition); src.read(imgHeader)

const phys = makeChunkPhys(dpi)

const imgBody = new Uint8Array(src.capacity - insertPosition); src.read(imgBody)

// if (!src.empty()) throw new Error('Something is wrong.')

return new Uint8Array([

...Array.from(imgHeader),

...Array.from(phys),

...Array.from(imgBody)

])

}

code:mod.ts

// Number of pixels per unit when DPI is 72

// Number of pixels per unit when devicePixelRatio is 1

const PX_PER_METER = 2835

const bytes = (num, byteLength): Array<number> => {

const binStr = num.toString(2).padStart(byteLength * 8, '0')

const binArr = binStr.match(/\d{8}/g)

return binArr.map(v => parseInt(v, 2))

}

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

const _buf = new Buffer(buf.bytes())

const {dpi} = await parsePngFormat(_buf)

// 既存のpHYsを上書きしない

if (dpi !== undefined) return -1

// すべて既読、つまり、"IDAT"が存在しない

if (_buf.empty()) return -1

// 既読bytes数

const readBytesNum = _buf.capacity - _buf.length

// chunkLengthと"IDAT"ぶんだけ戻した位置

return readBytesNum - (4 + 4)

}

const makeChunkPhys = (dpi: number): Uint8Array => {

const type = 'pHYs'.split('').map(c => c.charCodeAt(0))

const devicePixelRatio = dpi / 72

const pixelsPerMeter = Math.floor(PX_PER_METER * devicePixelRatio)

const data = [

...bytes(pixelsPerMeter, 4),

1 // meter

]

const pHYsChunk = [

0, 0, 0, 9, // 9 bytes

...type,

...data,

...bytes(crc(...type, ...data), 4)

]

return new Uint8Array(pHYsChunk)

}