Nostalgia Label

Emulating ZX Spectrum graphics in JavaScript

TL;DR: I'm using EcmaScript 6 Proxy objects to keep track of dirty blocks in emulated ZX Spectrum video RAM. Try it out at or dive into the code by cloning or forking

Emulating 1982 video RAM in JavaScript

Animated GIF screenshot made using It's no secret that I have a sweet spot for the Sinclair ZX Spectrum. One of the things I was amazed by as an eight-year-old was the (then) incredible 256×192 pixel colour graphics. Using only 6.75 kilobytes of video RAM, the custom Ferranti ULA chip pieced together the video signal 50 (or 60) times per second.

Software emulation of the Ferranti ULA has been done a lot of times, but reinventing the wheel is a great way of learning new (or old) things, so I decided to make an attempt of my own.

JavaScript, CANVAS and Proxies, oh my!

First of all, I'm using a CANVAS element and the CanvasRenderingContext2D object to draw graphics in the browser's window. I'm also using a Uint8ClampedArray to store the 6912 bytes of raw video RAM. For a more detailed description of the memory layout, scroll down a little. Each byte of the array corresponds exactly to one byte of ZX Spectrum RAM, so changing the contents of a single byte should trigger a redrawing of at least a part of the canvas.

I decided to redraw the canvas in blocks of 8×8 pixels, because this is close to how the ZX Spectrum ULA worked. Changing any one of the 8 bitmap bytes inside a block, or its attribute byte, should mark that block as "dirty" and when the next animation frame comes along, all dirty blocks should be rerendered. Because of this, there is also a Uint8Array of length 728 (32×24) keeping track of dirty blocks, so that I don't have to redraw all blocks every frame.

Using a Proxy object, I'm able to use the array normally, while correctly marking dirty blocks as needed. Without a Proxy, I would have to expose a setter method for changing the RAM contents.

// Without a Proxy object:
data.set(address, newValue);

// With a Proxy object:
data[address] = newValue;

The Uint8ClampedArray and Proxy construction looks like this:

var data = new Uint8ClampedArray(6912);

var dataProxy = new Proxy(data, {
    "set" : function(target, property, value, receiver) {
        if (property >= 0 && property < 6912) {
            data[property] = value;

            var dirtBlockIndex;

            if (property >= 6144) {
                // The index is inside the attribute section
                dirtBlockIndex = property - 6144;
            } else {
                // The index is inside the bitmap section
                dirtBlockIndex = blockIndexFromOffset(property);

            dirtyBlocks[dirtBlockIndex] = 1;

            return true;

        // Not a numeric index inside the boundaries
        return false;

This creates a Proxy that, when written to, sets the value of the hidden array, calculates what block is changed, and marks that block as dirty, so that the next call to the renderer only redraws the dirty blocks. This speeds up the rendering process a lot.

The ZX.Spectrum.Bitmap object exposes the following public functions:

  • poke(address, value): Changes one byte of video RAM (valid adresses are within the 16384..23295 range)
  • peek(address): Reads one byte of video RAM
  • ink(value): Sets the current INK colour (0..7)
  • paper(value): Sets the current PAPER colour (0..7)
  • bright(value): Sets the current BRIGHT value (0..1)
  • flash(value): Sets the current FLASH colour (0..1)
  • cls(): Clears the screen using the current settings
  • plot(x, y): Sets one pixel, affecting the colour block
  • unplot(x, y): Clears one pixel, affecting the colour block
  • line(x1, y1, x2, y2): Draws a one pixel line, affecting colour blocks

Try it out

Go to and try it out for yourself. You can also clone or fork the code and play around. Pull requests are more than welcome!

Back to the 1980s

The ZX Spectrum was an amazing computer for its time. An advanced BASIC interpreter fit snugly into 16 kilobytes of ROM, and the 48 kilobytes of RAM included 6.75 kilobytes of graphics memory. Using BASIC commands like PLOT, INK and CIRCLE, you could write algorithms to draw things of beauty on the screen, but you had to look out for attribute clash.

The video RAM consisted of monochrome bitmap data containing one bit per pixel for a total of 256×192=49152 bits, fitting into 49152/8=6144 bytes, starting at address 16384. The order of pixel rows inside this memory area is a little strange, as rows are not placed linearly (each line of 256 pixels is not exactly 256 bits after the one above it). To calculate the screen address of the first pixel of a Y coordinate, you encode the address as follows:


This effectively divided the screen vertically into three blocks of 256×64 pixels, within which it is easy to get to the next line of characters, and also easy to get to the next line within a character block by simply adding one to the high byte of the address, but calculating the screen position from a pixel coordinate is really convoluted.

Directly after that monochrome bitmap, at address 22528, was one attribute byte per 8×8 block, containing the colour values for the "ones" and "zeros" of the bitmap data. Each attribute byte is encoded like this:

  • F holds a one for FLASH mode, where INK and PAPER alternates every 32 frames
  • B holds a one for BRIGHT mode, where both INK and PAPER are a little brighter
  • P0..P2 holds a value between 0 and 7 for the PAPER colour, which is used for zeroes in the bitmap
  • I0..I2 holds a value between 0 and 7 for the INK colour, which is used for ones in the bitmap

Avoiding the "attribute clash" was tricky, and you had to really plan your artwork or your graphics algorithm to make sure that you only ever needed two distinct colours inside each 8×8 block of pixels. Artist Mark Schofield wrote an article in 2011, describing his process of planning and creating a piece of ZX Spectrum artwork.

If you are looking for more ZX Spectrum art, here are a couple of sites you might have a look at:

Posted by Anders Tornblad on Category JavaScript Labels
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Artsy part 3, SoundBox

Wow, it's been a really long time since I wrote anything about my demo projects here. I apologize for that, but the good news is that I have finally found some extra time to spend on "pleasure programming" again.

First, there's the unfinished business of Artsy (and slightly insane). About five months ago, I was finished with the first two parts, and I haven't had time to start on part three until now. So far, it looks pretty good. Unfortunately it doesn't yet work on iOS devices, but feel free to check it out:

Artsy part 3


Someone in the CODEF Facebook group linked to the SoundBox tool, and I find it quite amazing. It's a JavaScript-based chip tune tracker, with a really nice and small playback routine. I have been experimenting a bit with it, and here are some results:

Lots of unfinished business in this update, and I'm sorry about that.

Posted by Anders Tornblad on Category JavaScript Labels
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Artsy (and slightly insane), code now public

I just made the Artsy repository public. You can check out all my commits from the first boilerplate code to the current two-part demo beta on github at /lbrtw/artsy.

In a couple of weeks I will continue adding the third part of the demo. Stay tuned! :)

Posted by Anders Tornblad on Category JavaScript Labels
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Artsy (and slightly insane), first two parts now in beta

A couple of minutes ago, I uploaded a new version of Artsy (and slightly insane). It includes JavaScript remakes of the first two parts of the iconic Amiga Demo Arte by Sanity.

The code is pure JavaScript and Canvas. There is no Flash, no Silverlight, no WebGL stuff, and there are no frameworks involved. When I'm done remaking the third and final part of Arte, my plans are to release the full source code for the demo. Also, I'll take the "plumbing" parts of the code and release as a JavaScript demo framework in itself, and I'll open-source it.

But for now, enjoy the first two parts of "Artsy (and slightly insane)". The adress is

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Phenomenal & Enigmatic, part 4

TV Cube remake I remember seeing the "TV Cube" part of Enigma for the first time – and not really being able to figure out how it was made. Heck, I couldn't even do the math for a proper backface culling, so back in the 1990s my occational 3D objects were never any good. So the thought of making 2D and 3D objects appear on the surfaces of another 3D object was way beyond my understanding of math.

Once again, I am aware that the prettier way of doing this is by manipulating a transformation matrix to rotate, translate and project coordinates from different branches of a hierarchical coordinate system. But I ignored that and rolled it all by hand.

Star field

The stars on the front of the cube might look as if there is some depth, but that's just an illusion. Each star has an (X,Y) coordinate, and a third constant (which I called Z) that governs speed along the X axis and also the alpha component of its color. The lower the speed, the dimmer the light. When observed face on, it gives the impression of a 3D space, but it's really just a form of parallax scroller.

Pseudo code

// Star field
for (var star, i = 0; star = stars[i++];) {
    // Move the star a bit to the "right"
    star.x += (star.z * star.z * speedConstant);

    // Limit x to (-1 .. 1)
    if (star.x > 1) star.x -= 2;

    // Left out: Project the star's coordinates to screen coordinates
    var screenCoords = ( /* left out */ );

    // Draw the star, using Z to determine alpha and size
    context.fillStyle = "rgba(255,255,255," + (star.z * star.z).toFixed(3) + ")";
    context.fillRect(screenCoords.x, screenCoords.2, star.z * 2, star.z * 2);

Hidden line vector

Back in the days, I could never do a proper hidden line vector, because I didn't know how to properly cull back-facing polygons. For the Phenomenal & Enigmatic "TV Cube" part, I arranged all polygons in the hidden line pyramid so that when facing the camera, each polygon is to be drawn clockwise. That way I could use a very simple algorithm to determine each polygon's winding order.

I found one really efficient algorithm on StackOverflow, and I learned that since all five polygons are convex (triangles cannot be concave, and the only quadrangle is a true square), it's really enough to only check the first three coordinates, even for the quadrangle.

Rotating the pyramid in 3D space was exactly the same as with the intro part of the demo, and after all coordinates are rotated, I simple use the polygon winding order algorithm to perform backface culling, then drawing all polygons' outlines. Voilá, a hidden line vector.

Pseudo code

///Hidden line vector
// Points
var points = [
    { x : -40, y : -40, z :  70 }, // Four corners at the bottom
    { x :  40, y : -40, z :  70 },
    { x :  40, y :  40, z :  70 },
    { x : -40, y :  40, z :  70 },
    { x :   0, y :   0, z : -70 }  // And finally the top

// Each polygon is just an array of point indices
var polygons = [
    [0, 4, 3], // Four triangle sides
    [1, 4, 0],
    [2, 4, 1],
    [3, 4, 2],
    [3, 2, 1, 0] // And a quadrangle bottom

// First rotate the points in space and project to screen coordinates
var screenCoords = [];

for (var point, i = 0; point = points[i++];) {
    screenCoords.push(rotateAndProject(point)); // rotateAndProject is left out

// Then go through each polygon and draw those facing forward
for (var polygon, i = 0; polygon = polygons[i++];) {
    var edgeSum = 0;
    for (var j = 0; j < 3; ++j) {
        var pointIndex = polygon[j];
        var pointIndex2 = polygon[(j + 1) % 3];

        var point = screenCoords[pointIndex];
        var point2 = screenCoords[pointIndex2];

        edgeSum += (point2.x - point.x) * (point2.y + point.y);

    if (edgeSum < 0) {
        // This polygon is facing the camera
        // Left out: Draw the polygon using screenCoords, context.moveTo and context.lineTo

Plane vector

The plane vector is super-simple. Just rotating a plane around its center and then using the code already in place to project it to screen coordinates.


The function responsible for translating coordinates in the 3D space to screen coordinates is not particularly complex, since it's basically the exact same thing as for the intro part of the demo. Also, to determine which faces of the cube that are facing the camera, I just used the same backface culling algorithm as for the hidden line vector. I was really pleased with the end result.

The entire demo, including non-minified JavaScript, is available on GitHub: /lbrtw/enigmatic

Posted by Anders Tornblad on Category JavaScript Labels
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Artsy (and slightly insane), first part now in beta

In between writing about the Phenomenal & Enigmatic JavaScript demo, I'm also doing a JavaScript remake of the Arte demo by Sanity from 1993. The first effect I made was the "disc tunnel" effect, seen 2min 9sec into the YouTube clip, and the entire first part is now live, but still in beta.

The address is, but I haven't tested it that many browsers and devices yet. I do know that it crashes on Windows Phone 7.8 after just a few scenes, but it works really nicely on my iPad 3, especially in fullscreen mode. Add a shortcut to your iPad's start screen for fullscreen mode.

I will make some changes to the bitmap tunnel effect, and make sure that the demo runs correctly on most browsers and devices. Also, stay tuned for parts 2 and 3 of Sanity Arte, and of course there will be a blow-by-blow description here on when the whole thing is complete.

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Phenomenal & Enigmatic, part 3

When I made Phenomenal & Enigmatic, I didn't want to reuse any of the graphics art from the original demo. I had decided to take the music and some sense of the overall demo design, but didn't want to infringe on the original graphics artist's creativity, so for the Enigmatic logo, I turned to rendering the logo using code.

Original Enigma logotype One popular technique of creating good-looking logos back in the Amiga days was to first draw the logo flat, then duplicating it in a new layer a few pixels off, making that new layer translucent. Then one would paint in side surfaces and edge lines with varying levels of opacity. After adding some final touches, like surface textures or lens flares, the end result would be a glossy, glassy look, much like the original Enigma logo by Uno of Scoopex.

Enigmatic logotype The logo scene uses the same technique, painting the front, back and sides of the word ENIGMATIC as filled polygons with slightly different colors and opacity levels. During the scene, I animate some of the transformation vectors for effect. Of course, the original artwork by Uno is much better in exactly every way, but it was a fun exercise.


// Logo renderer
function transformLogoCoord(chapterId, time, x, y) {
    // Left out: Perform a simple coordinate transformation
    return { x : transformedX, y : transformedY };

function logoMoveTo(chapterId, time, x, y, xOffset, yOffset) {
    var coords = transformLogoCoords(chapterId, time, x, y);
    context.moveTo(coords.x, coords.y);

function logoLineTo(chapterId, time, x, y, xOffset, yOffset) {
    var coords = transformLogoCoords(chapterId, time, x, y);
    context.lineTo(coords.x, coords.y);

function renderLogo(chapterId, time) {
    var xOffset, yOffset;
    // Left out: Calculate xOffset and yOffset from the chapterId and time values

    // Draw bottom surfaces
    for (var block, i = 0; block = logoPolygons[i++];) {
        logoMoveTo(chapterId, time, block[0].x, block[0].y, 0, 0);
        for (var coord, j = 1; j = block[j++];) {
            logoLineTo(chapterId, time, coord.x, coord.y, 0, 0);
        logoLineTo(chapterId, time, block[0].x, block[0].y, 0, 0);

    // Left out: draw side surfaces

    // Left out: draw top surfaces

The entire demo, including non-minified JavaScript, is available on GitHub: /lbrtw/enigmatic

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Phenomenal & Enigmatic, part 2

Stars and text intro

Phenomenal Phenomena The opening scene of Enigma by Phenomena starts out looking like an average side-scrolling star parallax, which was very normal in 1991. Nice way to lower people's expectations. :) But after just a couple of seconds, the stars begin twisting and turning in space around all three axes.

Back in 1991 I knew how to rotate a plane around the origo, by simply applying the angle sum identities of Sine and Cosine. I also realized that rotating any 3D coordinate in space could by done by simply the case of rotating around more than one axis, one axis a time.

Flash-Free enigmatic In the Phenomenal & Enigmatic demo, I only rotate the stars around two axes. First I rotate the (x,z) components around the Y axis to get (x',z'), and then the (y,z') components around the X axis to get (y',z''). I also translate each star along the X axis before the rotation takes place. To finally get the 2D screen coordinates of the 3D space coordinate, I take the (x',y') coordinate and multiply by (2/(2+z'')) for a pseudo-distance feel. The z'' value controls both the color alpha component, and the size of the rectangle being drawn.

The even better way of doing this is through vector addition and multiplication, but I'm sticking to the math that I know. :) After all this math is in place, the trick is to change the offset and rotation variables in a nice way.

Rendering text is just a couple of calls to the context.fillText method and animating the value of context.globalAlpha.

Pseudo code

// Prefetch sin and cosine of angles
var cosY = Math.cos(yAngle);
var sinY = Math.sin(yAngle);
var cosX = Math.cos(xAngle);
var sinX = Math.sin(xAngle);

for (var star, i = 0; star = stars[i++]; ) {
    // Fetch x, y, z and translate x
    var x = star.x + xOffset;
    var y = star.y;
    var z = star.z;

    // Limit x to [-1 .. 1]
    while (x > 1) x -= 2;
    while (x < -1) x += 2;

    // Rotate (x, z) around Y axis
    var x2 = x * cosY + z * sinY; // x'
    var z2 = z * cosY - x * sinY; // z'

    // Rotate (y, z') around X axis
    var y2 = y * cosX + z2 * sinX; // y'
    var z3 = z2 * cosX - y * sinX; // z''

    // Transform to screen coordinates
    var screenX = x2 * 2 / (2 + z3) * halfScreenWidth + halfScreenWidth;
    var screenY = y2 * 2 / (2 + z3) * halfScreenWidth + halfScreenHeight;

    // Draw the star
    context.fillRect(screenX, screenY, 2 - z3, 2 - z3);

The entire demo, including non-minified JavaScript, is available on GitHub: /lbrtw/enigmatic

Posted by Anders Tornblad on Category JavaScript Labels
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Phenomenal & Enigmatic, part 1

Twenty-two years ago, the Amiga demo scene was extremely active. The boundaries of the little 16-bit miracle machine were stretched a little bit more for each new release, and a special breed of programmers was created. We loved efficient algorithms, we enjoyed squeezing out as much as possible from every CPU clock cycle, and we really liked showing off to each other.

Back then, I was a decent 68000 assembler programmer, but nowhere near being among the greatest. I knew my way around the Copper and the Blitter, I knew how trigonometry and vector mathematics worked for creating 3D worlds, and I understood that the "shadebobs" effect on the Amiga was nothing more than repeated full-adders, using the "Fat Agnus" chip's dedicated memory block manipulation instruction set.

My favorite demo from 1991 was Enigma by Phenomena, programmed by Olof "Azatoth" Lindroth, with music by the amazing Jimmy "Firefox" Fredriksson and Robert "Tip" Österbergh. The combination of music and direction with some really good programming set a new standard for demos on the Amiga.

First attempt

About four years ago, I started replicating the Enigma demo in C# and Silverlight 2, just as a side-project. I got as far as the opening scene and the "TV Cube" effect, which I must say I really nailed! But then I grew tired of the effort, and put the whole project aside. It just wasn't rewarding enough, but I did re-awaken some of my old "hacking the bare metal" programming skills.

Come 2013

For the last couple of weeks, I've been working from scratch, exploring what is possible using just a HTML5 AUDIO element, a CANVAS element, and a truck-load of JavaScript. Instead of trying to recreate the exact Enigma experience, I "borrowed" the amazing music, and did something of my own, inspired by Enigma.

I'll write a bit about each scene in the following weeks, but for now you're welcome to check out the fruit of my effort at

The entire demo, including non-minified JavaScript, is available on GitHub: /lbrtw/enigmatic

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