Difference between revisions of "Flags with Repeats"

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{{flags-snippet}}
{{flags-snippet}}
In these exercises you can use a function '''star''' which draws a five pointed star on the canvas.
You can control the position of the star by calling the '''transform''' method of ctx.
==Vietnam==
<div class=qu data-width="200" data-height="150">
The flag of Vietnam has a yellow five pointed star on a red
background. This star is a popular device on many flags, we define
the star once and use it many times in later flags.
The given star has a radius of 50, it is centred on 0,0
which is the wrong place. We must translate to put it in the
right place.
We can use the [https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/translate translate] method to move the star to the right location.
The translate call is to the wrong location (50,50) - it should be at
(100,75).
[[Image:flagvietnam.png]]
<pre class=usr>
function drawFlag(ctx)
{
  ctx.fillStyle = 'red';
  ctx.translate(50,50);
  star(ctx);
}
function star(ctx)
{
  ctx.beginPath();
  [ [ 0,-50],[11,-16],[48,-16],[18,6],[30,41],
    [ 0,18],[-30,41],[-18,6],[-48,-16],[-11,-16]
  ].forEach((xy) => ctx.lineTo(xy[0],xy[1]));
  ctx.fill();
}
</pre>
<pre class=ans>
function drawFlag(ctx)
{
  ctx.fillStyle = 'red';
  ctx.fillRect(0,0,200,150);
  ctx.translate(100,75);
  ctx.fillStyle='yellow';
  star(ctx);
}
function star(ctx)
{
  ctx.beginPath();
  [ [ 0,-50],[11,-16],[48,-16],[18,6],[30,41],
    [ 0,18],[-30,41],[-18,6],[-48,-16],[-11,-16]
  ].forEach((xy) => ctx.lineTo(xy[0],xy[1]));
  ctx.fill();
}
</pre>
</div>

Revision as of 08:42, 10 August 2021

<div style='background:silver;padding:5px'>
<canvas id='usr' width=--snippet-w-- height=--snippet-h--></canvas>
<canvas id='ans' width=--snippet-w-- height=--snippet-h-- style='display:none'></canvas>
</div>
--snippet-usr--
drawFlag(document.getElementById('usr').getContext('2d'));
(()=>{
  --snippet-ans--
  drawFlag(document.getElementById('ans').getContext('2d'));
})();
let [a,b] = ['usr','ans']
    .map(id=>document.getElementById(id))
    .map(elem=>elem.getContext('2d').getImageData(0,0,elem.width,elem.height))
let diff = a.data.map((v,i) => v===b.data[i]?1:0).reduce((acc,v)=>acc+v,0)*100/a.data.length;
let fb = document.createElement('div');
fb.innerText = `Score: ${diff.toFixed(1)}`;
document.body.appendChild(fb);
fetch(`/reportProgress.php?uid=--snippet-uid--&qid=--snippet-qid--&score=${diff}`);

In these exercises you can use a function star which draws a five pointed star on the canvas.

You can control the position of the star by calling the transform method of ctx.

Vietnam

The flag of Vietnam has a yellow five pointed star on a red background. This star is a popular device on many flags, we define the star once and use it many times in later flags.

The given star has a radius of 50, it is centred on 0,0 which is the wrong place. We must translate to put it in the right place. We can use the translate method to move the star to the right location.

The translate call is to the wrong location (50,50) - it should be at (100,75).

Flagvietnam.png

function drawFlag(ctx)
{
  ctx.fillStyle = 'red';
  ctx.translate(50,50);
  star(ctx);
}

function star(ctx)
{
  ctx.beginPath();
  [ [ 0,-50],[11,-16],[48,-16],[18,6],[30,41],
    [ 0,18],[-30,41],[-18,6],[-48,-16],[-11,-16]
  ].forEach((xy) => ctx.lineTo(xy[0],xy[1]));
  ctx.fill();
}
function drawFlag(ctx)
{
  ctx.fillStyle = 'red';
  ctx.fillRect(0,0,200,150);
  ctx.translate(100,75);
  ctx.fillStyle='yellow';
  star(ctx);
}

function star(ctx)
{
  ctx.beginPath();
  [ [ 0,-50],[11,-16],[48,-16],[18,6],[30,41],
    [ 0,18],[-30,41],[-18,6],[-48,-16],[-11,-16]
  ].forEach((xy) => ctx.lineTo(xy[0],xy[1]));
  ctx.fill();
}