Drawing lines with canvas by using for loop

Question

I am trying to draw lines with canvas and I am changing the coordinates with a for loop.

here is my canvas element:

Answer 1

That's again the eternal issue of forgetting to call beginPath.
Each time you call moveTo then lineTo, you create a new *sub*path, which adds to the current Path.
Then each time you call stroke(), the current path, so all the current subpaths get re-drawn, when the last added path is drawn for the first time.
Since opacities will add-up, top lines will reach 100% opacity (alpha=255) when the bottom line, drawn once, will have a 20% opacity (lineWidth=0.2) .

In your second fiddle, you stroke only once, so all lines have 20% opacity, which is correct for the 0.2 lineWidth.

So : use beginPath before drawing a new figure.
In this case you have two choices :
• draw line by line OR
• draw once a path with all lines as subpath.

(see code below).

TIP : To get clean lines remember that pixels's center is at the (+0.5, +0.5) coordinates of each pixels, so a 'trick' is to translate by 0.5, 0.5 on app start, then only use rounded coordinates and lineWidth.

1) draw line by line

http://jsfiddle.net/gamealchemist/J6zzD/6/

var c = document.getElementById('c');
var ctx = c.getContext('2d');
ctx.translate(0.5, 0.5);
ctx.lineWidth = 1;

for (var y = 18; y < 300; y += 18) {
    strokeLine(ctx, y);
}

function strokeLine(ctx, y) {
    ctx.beginPath();
    ctx.strokeStyle = 'red';
    ctx.moveTo(0, y);
    ctx.lineTo(300, y);
    ctx.stroke();
}

2) draw multiple subPath : (you can have only one color for one stroke() )

http://jsfiddle.net/gamealchemist/J6zzD/7/

var c = document.getElementById('c');
var ctx = c.getContext('2d');
ctx.translate(0.5, 0.5);
ctx.lineWidth = 1;

ctx.strokeStyle = 'red';

ctx.beginPath();
for (var y = 18; y < 300; y += 18) {
    addLineSubPath(ctx, y);
}
ctx.stroke();

function addLineSubPath(ctx, y) {
    ctx.moveTo(0, y);
    ctx.lineTo(300, y);
}