I am trying to draw lines with canvas and I am changing the coordinates with a for loop.
here is my canvas element:
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);
}
See: https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Canvas_tutorial/Applying_styles_and_colors#A_lineWidth_example
Because canvas coordinates do not directly reference pixels, special care must be taken to obtain crisp horizontal and vertical lines.
Basically, because you're trying to draw a line that's 0.2 pixels wide, the browser does some math to approximate a continuous number into discrete units and you get your "fading" lines.
So now we can fix up your code by changing context.lineWidth
to 1 (I actually remove it because it defaults to 1) and shifting everything down by half a pixel.
var c = document.getElementById('c');
ci = c.getContext('2d');
for(var a = 18.5; a < 300.5; a +=18)
{
fnc(a, ci);
}
function fnc(x, ci)
{
ci.strokeStyle = 'red';
ci.moveTo(0, x);
ci.lineTo(300, x);
ci.stroke();
}
Demo