HTML5 Canvas Drawing in Real Time

Question

Question: How can I make putImageData() update the canvas in real time, as various parts of the image have been computed?

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I am working on a JavaScript/TypeScript application to draw the Mandelbrot set on an HTML5 <canvas> element. Math and details aside, my application draws the set just fine. However, if you are familiar with visualizing the set, you know that it can take a long time to draw.

It will draw in a few seconds, but until then, the canvas is completely blank, then the image appears. I'm looking for a way to draw each row as it is computed using putImageData(). Here is what I am trying:

// part of the class definition
private Context: CanvasRenderingContext2D;
private ImageData: ImageData;
private Pixels: number[];

constructor() {
    var c: HTMLCanvasElement = <HTMLCanvasElement>document.getElementById("can");

    this.Context = c.getContext("2d");
    this.ImageData = this.Context.createImageData(this.Size.Width, 1);
    this.Pixels = this.ImageData.data;
}

public draw() {
    // tried this... does not help
    // var handler = function(m: Mandelbrot) {
    //    m.Context.putImageData(m.ImageData, 0, i)
    // };

    for(var i: number = 0; i < this.Size.Height; ++i) {    // Loop over each row
        for(var j: number = 0; j < this.Size.Width; ++j) { // Calc px. for one row
            // all the math to compute the set... (works)

            this.setPixelColor(j, color); // sets a color in this.Pixels (works)
        }

        // setTimeout(handler(this), 0); // does not help
        this.Context.putImageData(this.ImageData, 0, i); // Draw the row on the canvas?
    }
}

Somehow, the putImageData() function, which is called after a row in the image has been computed, only shows the image after the entire image has been generated.

How can I make putImageData() update the canvas in real time, as each row has been computed?

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Latest update of non-working code:

var handler = function(m: Mandelbrot, i: number) {
    for (var j: number = 0; j < m.Size.Width; ++j) {
        // math

        m.setPixelColor(j, color);
    }

    m.Context.putImageData(m.ImageData, 0, i);
};

var that: Mandelbrot = this;

for(var i: number = 0; i < this.Size.Height; ++i) {
    setTimeout(function() {
        handler(that, i)
    }, 0);
}

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Working code, thanks to ekuusela:

var handler = function(m: Mandelbrot, i: number) {
    return function() {
        for (var j: number = 0; j < m.Size.Width; ++j) {
            // math

            m.setPixelColor(j, color);
        }

        m.Context.putImageData(m.ImageData, 0, i);
    }
};

for(var i: number = 0; i < this.Size.Height; ++i) {
    setTimeout(handler(this, i), 0);
}

Answer 1

Try wrapping putImageData and the calculation for a single row to a setTimeout call to execute it asynchronously (post accept edit: see the final code in the question, this won't work since i will be undefined in the putImageData row)

public draw() {
    var that = this;
    var drawRow = function() {
            for(var j: number = 0; j < that.Size.Width; ++j) { // Calc px. for one row
                that.setPixelColor(j, color); // sets a color in this.Pixels (works)
            }

            // TODO specify the dirty region in this call
            that.Context.putImageData(that.ImageData, 0, i); // Draw the row on the canvas?
        };
    for(var i: number = 0; i < this.Size.Height; ++i) {    // Loop over each row
        setTimeout(drawRow, 0);
    }
}