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First part of my application loads picture with file chooser from my file system and draws image to canvas. I take that canvas and convert it to data URL and also save it in my database. 

var imgData = canvas.toDataURL("image/jpg");
factory.saveData(imgData);                      // execute some java code

Here is the problem. I can't redraw that image, my javascript:

var pic = new Image();
pic.onload = function() {
    ctx.drawImage(pic, 0, 0);                  // ctx = canvas.getContext("2d")
};
// This data url is copy/pasted from database for simplicity
pic.src = "data:image/png;base64,iVBOR..........ElFTkSuQmCC";

Onload function is called but I just get transparent image.

milos
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    have you tried setting the width and height specifically, e. g. `ctx.drawImage(pic, 0, 0, canvas.width, canvas.height)`? – Simon Hänisch Jun 24 '18 at 23:42
  • @SimonHänisch yes, I tried different values and image is still transparent – milos Jun 25 '18 at 00:03
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    When do you call canvas.toDataURL? Are you sure the `pic.onload` handler has already fired at this time? i.e are you sure something has been drawn on the canvas when you do export its content? – Kaiido Jun 25 '18 at 01:17
  • @Kaiido I think you are right! `canvas.toDataUrl()` is called before anything is drawn to canvas. Brb trying to fix it. – milos Jun 25 '18 at 01:27

1 Answers1

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Probably has something to do with the fact that getDataFromDatabase() is asynchronous so you're returning a promise object to pic.src instead of the data url. Fix this by using a then call on getDataFromDatabase and using the result on pic.src like this:

var pic = new Image();
var pic.onload = function() {
    ctx.drawImage(pic, 0, 0);
};
getDataFromDatabase().then(dataUrl => {
    pic.src = dataUrl;
});

Here's a full example of this in action. I draw to a canvas, store the canvas data as a base64 data url into a variable, save it into a mock DB using closures, then reload the data from a simulated asynchronous function call by using a promise.

const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');

// Initialize canvas to a random image
const imageUrl = 'https://cdn.sstatic.net/Sites/stackoverflow/company/img/logos/so/so-logo.svg?v=a010291124bf';
const initialImage = new Image();
initialImage.src = imageUrl;
initialImage.onload = function() {
  ctx.drawImage(initialImage, 0, 0);
}

// Convert canvas to base64 data url
const imageData = canvas.toDataURL('image/png');
const getDataFromDatabase = saveData(imageData);

// Clear the canvas
ctx.clearRect(0, 0, canvas.width, canvas.height);

// Redraw the image
const pic = new Image();
getDataFromDatabase().then(data => {
  pic.src = data;
});
pic.onload = function() {
  ctx.drawImage(pic, 0, 0);
}

function saveData(image) {
  return function getDataFromDatabase() {
    return new Promise(resolve => {
      resolve(image);
    });
  }
}
<canvas id="myCanvas">
  
</canvas>

Edit: It shouldn't really matter where you get the image data from. If the data is not malformed, it should draw to the canvas. Here's an example identical to the way you're loading image data (from a hardcoded data url):

const imageData = 'data:image/png;base64,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';

const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');

const image = new Image();
image.onload = function() {
  ctx.drawImage(image, 0, 0);
}
image.src = imageData;
<canvas id="myCanvas">
</canvas>

I suggest you check your image data using a base64 image decoder to see if your image data is malformed or not. If your data url is valid, then something else I can suggest is ensuring your canvas width and height are equal to or greater than the image's width and height or else it'll be hidden (if the image you're supplying has empty space / padding).

Jake Miller
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  • I may have omitted some important info in my post I apologize. I didn't really call `getDataFromDatabase()` I just pasted base64 data code in quotations marks, so no promise object there. Also, I loaded my picture with file chooser from my file system, not from web like in your example. – milos Jun 25 '18 at 00:22
  • I tried [this](https://codebeautify.org/base64-to-image-converter) site and I see transparent image. Did run [this](https://stackoverflow.com/a/31245864/5059176) code and it returns FALSE. I guess I have malformed data, probably gonna open another post thanks for help. – milos Jun 25 '18 at 00:58
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    @vesa Yeah your base64 image data is malformed then. I’ll help you sort it out. Let me create a StackOverflow chat and I’ll invite you. Hang on. – Jake Miller Jun 25 '18 at 01:19
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    If the image data was not correct, then the onload event would not fire. – Kaiido Jun 25 '18 at 01:28
  • @Jake Miller can't join, 2 more reputaion needed to join – milos Jun 25 '18 at 01:30
  • @Kaiido Exactly. Which is why his canvas isn't drawing most likely. His image doesn't load properly on a base64 image decoder so I'm assuming his base64 data url is malformed in some way. – Jake Miller Jun 25 '18 at 01:32
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    "*Onload function is called but I just get transparent image.*" The problem is that they call toDataURL when the canvas is still empty. – Kaiido Jun 25 '18 at 01:35
  • @Kaiido Hmm, that makes sense. I overlooked that sentence. – Jake Miller Jun 25 '18 at 01:37
  • @vesa Can you post your code where you draw an image from the file system to the canvas as well? Kaiido made a good point that you're probably creating your data url before the canvas has a chance to draw the image that you select from your file system. – Jake Miller Jun 25 '18 at 01:37
  • you solved the problem, thanks. My `canvas.toDataURL("image/png")` was outside of Onload function. I just put that line inside Onload function and image is correctly drawn. – milos Jun 25 '18 at 01:40