从 canvas 获取图像数据

Question

I'm trying to add a signature feature to a web app. Users should have the possibility to sign with a pencil on a touch screen device. I'm using HTML5 canvas, JavaScript and PHP to get things done. I found a script for both mouse and touch support.

The drawing on the canvas works fine. When I submit the form, the scribble will get cropped properly (so it seems). But the generated base64-png data (saved in a database) is an empty image.

Here's the JS code I'm using:

var canvas, ctx;
var mouseX, mouseY, mouseDown = 0;
var touchX, touchY;
var lastX, lastY = -1;

canvas = document.getElementById('scribble-canvas');
ctx = canvas.getContext('2d');

canvas.addEventListener('mousedown', scribbleMouseDown, false);
canvas.addEventListener('mousemove', scribbleMouseMove, false);
canvas.addEventListener('mouseup', scribbleMouseUp, false);

canvas.addEventListener('touchstart', scribbleTouchStart, false);
canvas.addEventListener('touchend', scribbleTouchEnd, false);
canvas.addEventListener('touchmove', scribbleTouchMove, false);


function drawLine(ctx, x, y, size) {

    if (lastX == -1) {
        lastX = x;
        lastY = y;
    }

    ctx.strokeStyle = "#000000";
    ctx.lineCap = "round";
    ctx.lineWidth = size;

    ctx.beginPath();
    ctx.moveTo(lastX, lastY);
    ctx.lineTo(x, y);
    ctx.stroke();
    ctx.closePath();

    lastX = x;
    lastY = y;
} 


function scribbleMouseDown() {
    mouseDown = 1;
    drawLine(ctx, mouseX, mouseY, 3);
}


function scribbleMouseUp() {
    mouseDown = 0;
    lastX = -1;
    lastY = -1;
}


function scribbleMouseMove(e) { 

    getMousePos(e);

    if (mouseDown == 1) {
        drawLine(ctx, mouseX, mouseY, 3);
    }
}


function getMousePos(e) {

    if ( ! e) {
        var e = event;
    }

    if (e.offsetX) {
        mouseX = e.offsetX;
        mouseY = e.offsetY;
    }

    else if (e.layerX) {
        mouseX = e.layerX;
        mouseY = e.layerY;
    }
}


function scribbleTouchStart() {

    getTouchPos();
    drawLine(ctx, touchX, touchY, 3);

    event.preventDefault();
}


function scribbleTouchEnd() {

    lastX = -1;
    lastY = -1;
}


function scribbleTouchMove(e) { 

    getTouchPos(e);
    drawLine(ctx, touchX, touchY, 3); 

    event.preventDefault();
}


function getTouchPos(e) {

    if ( ! e) {
        var e = event;
    }

    if (e.touches) {

        if (e.touches.length == 1) {
            var touch = e.touches[0];
            touchX = touch.pageX - touch.target.offsetLeft;
            touchY = touch.pageY - touch.target.offsetTop;
        }
    }
}


function crop(canvas) {
    var w = canvas.width, h = canvas.height;
    var pix = {x:[], y:[]};
    var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
    var x, y, index;

    for (y = 0; y < h; y++) {
        for (x = 0; x < w; x++) {
                index = (y * w + x) * 4;
 
             if (imageData.data[index+3] > 0) {
             pix.x.push(x);
             pix.y.push(y);
            } 
        }
    }

    pix.x.sort(function(a,b){return a-b});
    pix.y.sort(function(a,b){return a-b});

    var n = pix.x.length-1;
    
    w = 1 + pix.x[n] - pix.x[0];
    h = 1 + pix.y[n] - pix.y[0];
    var cut = ctx.getImageData(pix.x[0], pix.y[0], w, h);

    document.body.style.opacity = 0;

    canvas.width = w;
    canvas.height = h;
    ctx.putImageData(cut, 0, 0);
    
    return canvas.toDataURL();
}

On the PHP side I just pick the data returned by the crop() function and save it to the database.

What I tried so far is to strip off the touch support to see if it works just on mouse devices.

---

Additional code as suggested by user traktor:

The following jQuery code submits the form with the image data to the PHP script:

$(document).on('click', '#scribble-submit', function(e) {

    e.preventDefault();
    var btn = $(this);
    var data = crop(document.querySelector('#scribble-canvas'));

    $.post('/scribble/ax/ins_scribble', $('#form-scribble').serialize()  + '&data=' + data, function(ans) {

        window.location.href = '/' + btn.data('redirect');
    });
});

The PHP part looks like this (using the CI3 framework):

$post = $this->input->post();

$this->db->where('id', $id);
$this->db->update('personen', ['signatur' => $post['data']]);

echo TRUE;

So far, jQuery and PHP works fine. The data is successfully inserted into the database. The LONGBLOB field contains this code:

data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAMQAAAC3CAYAAABNEPEEAAAAAXNSR0IArs4c6QAAAERlWElmTU0AKgAAAAgAAYdpAAQAAAABAAAAGgAAAAAAA6ABAAMAAAABAAEAAKACAAQAAAABAAAAxKADAAQAAAABAAAAtwAAAAB1wSLaAAAYVElEQVR4Ae2dCZQeVZXHmyUhIYQ1gBAgHSDsCAxbWNPsO4ZBBBkwERxQcEARUUdQdJyj6DjA0VE2YdhBhs2ASAhDwo4gy4GwBmggILIZgbAKM79/893O7er1W r7arn3nH /W  reu/ef71Xb63qhdpCssbAMhg0CowBoyuh4iaDYeAeUI0szcnt4P9AJ/gbGEiGer6dN5/EZoG7wGvgzyC3slBuLe9tuArRSGAFSjdsCWA3bmv0RcFsMFih4JRuWQVNUIGaC14E1YpsWA8sAp4DSmtxsGQlXKoSynbFy 48yrsY/VfwPHgDqLKsAN4G/wVuAJmWVlUIFYC9we5AN78TJAvpROJUcPp7IlpBVwFSQV8ebABCssvAFzDtwuya19aWZoVYGceXBWPBumAXoEKsp8Z4sBoYDkL6Z AFfnoJPANWAu B/h4Q/NSn2IMjrS6TunXLgTXAYC2bWtf/BTeCaeBNkClpZIVYGM9EyilAXYyRoFVPbBWieeDlSvgB4afAh2A2SLZGRPUrjewyfUQufwQq6OpGqIvhQxV4HVsou/MievipQowB6goqVHnYAWwC1HXycg4Hvwb3 8hW6/VWiHE4sBFYC6wH9gbqugxVHuDE50AnSBbSicQN1mVS/ipUtwEVNPVbld7HICQ7DKgFOQkcmzDpKY6fANcCVZDcip4AVwENnlRo 8M7/Canp4PbwXXgSLAlEEkh5WFgPK7uB84CyfKibuAIkEuZjNV3AjX/Scfs Ap U8uhZjQkGPAMqFulnoTGE1ZeFP4eHAByI8OxdEdwA/COSL8UfA3sDDT9GRIMDMbAlzlBPYdXgZUnTR5MApmX1bDw6 A YMYrVK0OCQbqYeBfuNiXKc1CfgNktku9Lcb9DmiGxgx/EP1woBmYkGCgHgZW5eKDgR6uVr7sYauylzlJGipjJ2XOyjAo7wzshAO QkifAZbKkmMrYoymxczQu9HXz5KBYUuhGNgcbzQbaeVNocYai4GWywpYcDzwgx7NHIUEA2kysB2JPwx8pTgqzQyHmrb6b96oY4Z6YZwXDNTJwIZc/9/Ayp/WuzYGLZVvk7sZpMW1kGCgmQzsQGZW/hQ ALQG1hLR2EEbsMygE1piRWRadgb2hYCbgZXDa9C1s7mpYmOHl8nVDFm7qRZEZsHAAga iGrlUOFdQFP TZPPkpM3IMYOTaM MuqDAe2BOg68BqxcXt/HealFaQOeZawZppBgIAsM3IQRVi470XdvllHaeWgZq3KEBANZYOAIjHgMWNl8E33dtA0bSwbqo1mm/5R2hpF MFAFAxtwru86nVjFtTWd2sFVflv3hJpSiYuCgfQYUCWwB7YqhypJKjKCVH8ILDNt6AsJBrLGgLpJ6i5ZOX0cXd2phss4UtR dMtIc8AhwUAWGdCAuhNYWU3l4a1MLIMP0UOCgSwzoKlXK6 z0dsbYezCLpFxTo/ZJUdGqJlkQBXi7Ypl sDFXo228uckaDXuB41OPNILBlJg4GjStDL7CnpHvXn4FkI7DE003xsSDGSdAbUSeoFNsjzYvEtrwJ/9ScO/97BOA9KMJIKBZjDwTTKxVkIPcr22ULdcQAqW6Bz0JetOMRIIBprDQAfZ 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Using this data as an image source for the img tag results in the empty image.

Answer 1

The problem is this jQuery line:

$.post('/scribble/ax/ins_scribble', $('#form-scribble').serialize()  + '&data=' + data, function(ans) { 

Combining the serialize function and an addition of &data= didn't work. So I changed it to:

$.post('/scribble/ax/ins_scribble', { data: data }, function(ans) {

Answer 2

You code runs fine:

 var canvas, ctx; var mouseX, mouseY, mouseDown = 0; var touchX, touchY; var lastX, lastY = -1; canvas = document.getElementById('scribble-canvas'); ctx = canvas.getContext('2d'); canvas.addEventListener('mousedown', scribbleMouseDown, false); canvas.addEventListener('mousemove', scribbleMouseMove, false); canvas.addEventListener('mouseup', scribbleMouseUp, false); canvas.addEventListener('touchstart', scribbleTouchStart, false); canvas.addEventListener('touchend', scribbleTouchEnd, false); canvas.addEventListener('touchmove', scribbleTouchMove, false); function drawLine(ctx, x, y, size) { if (lastX == -1) { lastX = x; lastY = y; } ctx.strokeStyle = "#000000"; ctx.lineCap = "round"; ctx.lineWidth = size; ctx.beginPath(); ctx.moveTo(lastX, lastY); ctx.lineTo(x, y); ctx.stroke(); ctx.closePath(); lastX = x; lastY = y; } function scribbleMouseDown() { mouseDown = 1; drawLine(ctx, mouseX, mouseY, 3); } function scribbleMouseUp() { mouseDown = 0; lastX = -1; lastY = -1; } function scribbleMouseMove(e) { getMousePos(e); if (mouseDown == 1) { drawLine(ctx, mouseX, mouseY, 3); } } function getMousePos(e) { if (!e) { var e = event; } if (e.offsetX) { mouseX = e.offsetX; mouseY = e.offsetY; } else if (e.layerX) { mouseX = e.layerX; mouseY = e.layerY; } } function scribbleTouchStart() { getTouchPos(); drawLine(ctx, touchX, touchY, 3); event.preventDefault(); } function scribbleTouchEnd() { lastX = -1; lastY = -1; } function scribbleTouchMove(e) { getTouchPos(e); drawLine(ctx, touchX, touchY, 3); event.preventDefault(); } function getTouchPos(e) { if (!e) { var e = event; } if (e.touches) { if (e.touches.length == 1) { var touch = e.touches[0]; touchX = touch.pageX - touch.target.offsetLeft; touchY = touch.pageY - touch.target.offsetTop; } } } function crop(canvas) { var w = canvas.width, h = canvas.height; var pix = { x: [], y: [] }; var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height); var x, y, index; for (y = 0; y < h; y++) { for (x = 0; x < w; x++) { index = (y * w + x) * 4; if (imageData.data[index + 3] > 0) { pix.x.push(x); pix.y.push(y); } } } pix.x.sort(function(a, b) { return a - b }); pix.y.sort(function(a, b) { return a - b }); if (pix.x.length == 0) { return '#'; } var n = pix.x.length - 1; w = 1 + pix.x[n] - pix.x[0]; h = 1 + pix.y[n] - pix.y[0]; var cut = ctx.getImageData(pix.x[0], pix.y[0], w, h); //document.body.style.opacity = 0; canvas.width = w; canvas.height = h; ctx.putImageData(cut, 0, 0); return canvas.toDataURL(); } document.getElementById('scribble-submit').addEventListener('click', function(e) { e.preventDefault(); var data = crop(document.querySelector('#scribble-canvas')); document.getElementById('scribble-result').setAttribute('src', data); });

 <html> <body> <canvas id="scribble-canvas" style="width:200; height:200; border: 1px solid blue;"></canvas> <br> <button id="scribble-submit"> Run </button> <br> Output: <br> <img src="#" id="scribble-result" /> </body> </html>

You either send the wrong data or use different data to render.

If you are saving into a .png file, you may want to strip out the initial data:image/png;base64, before turning that into bytes.