Download Canvas With Javascript

I am currently testing using the element to draw all of the backgrounds (I will add effects later to these images later and is the reason I'm not using CSS to load the images). That said, I'm currently having difficulty loading a image on to the canvas. Here is the code:

This tutorial describes how to use the tag_hash_106 element to draw 2D graphics, starting with the basics. The examples provided should give you some clear ideas about what you can do with canvas, and will provide code snippets that may get you started in building your own content.

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Using the element is not very difficult, but you do need a basic understanding of HTML and JavaScript. The element is not supported in some older browsers, but is supported in recent versions of all major browsers. The default size of the canvas is 300 pixels 150 pixels (width height). But custom sizes can be defined using the HTML height and width property. In order to draw graphics on the canvas we use a JavaScript context object, which creates graphics on the fly.

Part of a web application I'm developing requires me to create bar graphs to display various information. I figured, if the user's browser is capable, I would draw them using the HTML5 canvas element. I have no problem drawing lines and bars for my graphs, but when it comes to labeling the axes, the bars, or the lines I ran into a snag. How do I draw rotated text onto a canvas element so that it lines up with the item it is labeling? A couple examples include:

Posting this in an effort to help others with similar problems. I solved this issue with a five step approach -- save the context, translate the context, rotate the context, draw the text, then restore the context to its saved state.

I think of translations and transforms to the context as manipulating the coordinate grid overlaid on the canvas. By default the origin (0,0) starts in the upper left hand corner of the canvas. X increases from left to right, Y increases from top to bottom. If you make an "L" w/ your index finger and thumb on your left hand and hold it out in front of you with your thumb down, your thumb would point in the direction of increasing Y and your index finger would point in the direction of increasing X. I know it's elementary, but I find it helpful when thinking about translations and rotations. Here's why:

When you translate the context, you move the origin of the coordinate grid to a new location on the canvas. When you rotate the context, think of rotating the "L" you made with your left hand in a clockwise direction the amount indicated by the angle you specify in radians about the origin. When you strokeText or fillText, specify your coordinates in relation to the newly aligned axes. To orient your text so it's readable from bottom to top, you would translate to a position below where you want to start your labels, rotate by -90 degrees and fill or strokeText, offsetting each label along the rotated x axis. Something like this should work:

We should still move the text by an amount to get it centered vertically, and unfortunately canvas does not have great line height support, so that's a guess and check thing ( correct me if there is something better ).

First of all, the height of the text is equal to the pixel font size.Now, this was something I read a while ago, and it has worked out in my calculations. I'm not sure if this works with all fonts, but it seems to work with Arial, sans-serif.

Why resize the canvas?I usually have a canvas that isn't appended to the page. I use it to draw all of my rotated text, then I draw it onto another canvas which I display.For example, you can use this canvas to draw all of the labels for a chart (one by one) and draw the hidden canvas onto the chart canvas where you need the label (context.drawImage(hiddenCanvas, 0, 0);).

IMPORTANT NOTE: Set your font before measuring your text, and re-apply all of your styling to the context after resizing your canvas. A canvas's context is completely reset when the canvas is resized.

I'm trying to flip/mirror an image as I paint it on an HTML canvas; I found a game tutorial showing a sprite sheet per direction a character has to face, but this doesn't seem quite right to me. Especially since each frame has a different size.

Here is a link to a fiddle I started. The fiddle won't work because you can't use cross domain images (womp womp), but you can still see my code thus far. When mousing over my canvas object I'm able to pixelate the image, but it's kind of backwards to what I'm attempting to get. Any help or advice would be greatly appreciated.

This way when you draw to a layer -- it actually goes to that "layer". Layers with transparent positions can be seen through to below layers (Canvases). Layer stacking is controlled with z-index property.

Using multiple canvases should be faster, because the canvas gets drawn off-screen and then just blitted to the screen by the browser. You put the burden of switching layers on the browser, which just has to move some rectangles of graphics data around.

Right now, I am using x=event.offsetX; to get the mouse position. This works very well in chrome, however when it comes to firefox, it doesn't work. I changed the code to x=event.layerX. but it seems that layerX is the position of my mouse relative to the web page, not the position of the canvas. because I always see an offset.

You will need a custom function to work out where the element is and then work out where the mouse is within that element. Here is an example. It uses this function from quirks mode and my JavaScript library which should not be difficult to translate into jQuery.

This will not work in IE due to it not supporting pageX. You will have to pass the event object through a function like this to correct that. But as 2x2p1p said, canvas is not supported by any Internet Explorer below version 9.

I've tried a few different ways that I have seen on here, but I can't quite get my image to move. Whenever I try adapting code for arrow key presses, it just seems to make my canvas shrink and my player model (spaceperson) disappear.

I am quite new to javascript, and I am just trying to work out how I can move the specperson image using arrow keys. I was trying to make a class for space person to access their x,y values, but I can't seem to draw the image without using .onload

Here's some slightly modified code from a game I was noodling around with a while back. If you want to see more code, check out the complete JS on GitHub. The game is incomplete but you should gather some helpful clues as to how to move an image around the canvas.

In the following code i want to rotate the text of each element of the array in javascript. If i use for example ctx.rotate(Math.PI/10) before the filltext, it rotates all the elements. The positioning of the text also changes with ctx.rotate(Math.PI/10) and if i use 90 degrees, ctx.rotate(Math.PI/2) the text does not show on the canvas.

As i said, i want to rotate each element on its own and with that the positioning of each element should stay the same as in the non-rotated text as in the code above. Thus each element has to rotate around its own axis. How can i achieve this?

NOTE: This has to do with how existing canvas elements are rendered when scaled up, not to do with how lines or graphics are rendered onto a canvas surface. In other words, this has everything to do with interpolation of scaled elements, and nothing to do with antialiasing of graphics being drawn on a canvas. I'm not concerned with how the browser draws lines; I care about how the browser renders the canvas element itself when it is scaled up.

Is there a canvas property or browser setting I can change programmatically to disable interpolation when scaling elements? A cross-browser solution is ideal but not essential; Webkit-based browsers are my main target. Performance is very important.

Of course, one could manually scale up images using nearest-neighbor interpolation onto high-resolution canvas surfaces in javascript, or even pre-scale images server-side, but in my case this will be forbiddingly costly so it is not a viable option.

A more difficult option is to use pixel manipulation and write an algorithm yourself for the job. Each pixel of the first image becomes a 5x5 block of pixels on the new image. It wouldn't be too hard to do with imagedata.

The first thing we need to do is to get the canvas element inside a variable. To do that we use the document.querySelector() method, which receives the element name ('canvas' in this case). Now we can interact with the canvas element from our javascript code.

There you go. What happened here is that we told the object FontFace to load our font and then we pass that font as a parameter of the function that will be executed next, the one inside the .load() method. Then we add the font to our document fonts and that's it, now we can display our "Hello, World!" with our custom font.

Now, all we need to do is to get our canvas context, assign the font we're gonna use, center the text, and draw our "Hello, World!" in the middle of our canvas. ff782bc1db