JavaScript + canvasで画像変換処理をするときのスニペット

var src = ctx.getImageData(0, 0, canvas.width, canvas.height)
var dst = ctx.createImageData(canvas.width, canvas.height)
for (let i = 0; i < src.data.length; i = i + 4) {
    var x = (src.data[i] + src.data[i + 1] + src.data[i + 2]) / 3
    dst.data[i] = dst.data[i + 1] = dst.data[i + 2] = x
    dst.data[i + 3] = src.data[i + 3]
}
ctx.putImageData(dst, 0, 0)

var src = ctx.getImageData(0, 0, canvas.width, canvas.height)
var dst = ctx.createImageData(canvas.width, canvas.height)
for (let i = 0; i < src.data.length; i = i + 4) {
    var x = (src.data[i] + src.data[i + 1] + src.data[i + 2]) / 3
    if (int_thresh > x) {
        x = 255
    } else {
        x = 0
    }
    dst.data[i] = dst.data[i + 1] = dst.data[i + 2] = x
    dst.data[i + 3] = src.data[i + 3]
}
ctx.putImageData(dst, 0, 0)

var src = ctx.getImageData(0, 0, canvas.width, canvas.height)
var dst = ctx.createImageData(canvas.width, canvas.height)
for (let i = 0; i < src.data.length; i = i + 4) {
    dst.data[i] = Math.abs(255 - src.data[i])
    dst.data[i + 1] = Math.abs(255 - src.data[i + 1])
    dst.data[i + 2] = Math.abs(255 - src.data[i + 2])
    dst.data[i + 3] = src.data[i + 3]
}
ctx.putImageData(dst, 0, 0)

var src = ctx.getImageData(0, 0, canvas.width, canvas.height)
var dst = ctx.createImageData(canvas.width, canvas.height)
var int_g = 2.0
const correctify = val => 255 * Math.pow(val / 255, 1 / int_g)
for (let i = 0; i < src.data.length; i += 4) {
    dst.data[i] = correctify(src.data[i])
    dst.data[i + 1] = correctify(src.data[i + 1])
    dst.data[i + 2] = correctify(src.data[i + 2])
    dst.data[i + 3] = src.data[i + 3]
}
ctx.putImageData(dst, 0, 0)

const cv = {
    grayscale: (src, dst) => {
        for (let i = 0; i < src.data.length; i = i + 4) {
            let x = (src.data[i] + src.data[i + 1] + src.data[i + 2]) / 3
            dst.data[i] = dst.data[i + 1] = dst.data[i + 2] = x
            dst.data[i + 3] = src.data[i + 3]
        }
    },
    threshold: (src, dst, int_thresh) => {
        int_thresh = int_thresh || 120
        for (let i = 0; i < src.data.length; i = i + 4) {
            let x = (src.data[i] + src.data[i + 1] + src.data[i + 2]) / 3
            if (int_thresh > x) {
                x = 255
            } else {
                x = 0
            }
            dst.data[i] = dst.data[i + 1] = dst.data[i + 2] = x
            dst.data[i + 3] = src.data[i + 3]
        }
    },
    reverse: (src, dst) => {
        for (let i = 0; i < src.data.length; i = i + 4) {
            dst.data[i] = Math.abs(255 - src.data[i])
            dst.data[i + 1] = Math.abs(255 - src.data[i + 1])
            dst.data[i + 2] = Math.abs(255 - src.data[i + 2])
            dst.data[i + 3] = src.data[i + 3]
        }
    },
    gamma: (src, dst, int_g) => {
        int_g = int_g || 2.0
        const correctify = val => 255 * Math.pow(val / 255, 1 / int_g)
        for (let i = 0; i < src.data.length; i += 4) {
            dst.data[i] = correctify(src.data[i])
            dst.data[i + 1] = correctify(src.data[i + 1])
            dst.data[i + 2] = correctify(src.data[i + 2])
            dst.data[i + 3] = src.data[i + 3]
        }
    }
}

const canvas = document.getElementById("canvas")
const default_imageurl = document.getElementById("img_src").value
drawImage(default_imageurl)

document.getElementById("menu").addEventListener("change", () => {
    try {
        let func_code = document.getElementById("menu").value || ""
        drawImage(default_imageurl, () => {
            let ctx = canvas.getContext('2d')
            let src = ctx.getImageData(0, 0, canvas.width, canvas.height)
            let dst = ctx.createImageData(canvas.width, canvas.height)
            cv[func_code](src, dst)
            ctx.putImageData(dst, 0, 0)
        })
    } catch (e) {
        alert(e)
    }
})

function drawImage(url, evt) {
    let ctx = canvas.getContext('2d')
    let image = new Image()
    image.src = url
    image.onload = () => {
        canvas.width = image.width
        canvas.height = image.height
        ctx.drawImage(image, 0, 0)
        evt()
    }
}