HTML5 Canvas stationary line draw - javascript

I have a canvas where I'm drawing a line using setInterval so it's animated. Right now it starts at 20,20 and end the animation at 200,20. Ideally I'd want the point on the canvas to never move, so it would look like the line is drawing but the canvas is moving. Best example would be a car that is driving that stays in frame while the background moves or think Mission Impossible where you follow the burning fuse as it burns.
is there a method to move the whole line? This way I can move the line to the left as I add to the line. If not would it be better to make a huge canvas and move it to the left as the line draws? What's the best way to create an animation like this?

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How to prevent drawing overlap (eraser) on HTML canvas?

Basically I have some movable shapes, and the ability to draw. When a user erases a drawing (drawing white over their drawing), I don't want them to be able to draw over the movable shapes.
Right now the only solution I can think of if to check every-time the eraser moves to see if it is colliding with a shape and just not draw white.
The problems I see with this solution:
Having to check every single shape every single time the eraser moves sounds like it will slow things down.
When I move the shapes around (and the canvas redraws) the white eraser drawings will appear over the shapes (so that part of the shape appears erased).

How to rotate and move a sprite towards the cursor?

Lets say that I have a player sprite, located at (player.x, player.y). On a mousemove event, I am capturing the cursor's position, which is (e.pageX, e.pageY). In the game, the player will be constantly moving, always towards the direction of the cursor. I am trying to find a way to to rotate the player sprite to always face the mouse cursor, and also move 10 pixels closer to the cursor every game tick. So far, this has me stumped. I have seen many examples of this online, and have tried all of them, but none seem to work. Any help that can be provided would be very much appreciated.
First of all, you must learn the basics of trigonometry. Without this knowledge, you will can't make something like you want.
Center of you image is the center of an circle. Knowing position of center and mouse position, you can calculate the angle. Calculated angle you can use to rotate image.

Canvas - Sticking png images together without taking into account transparent pixels

I have big horizontal strip image in photoshop which is made of lots of smaller elements. The background is transparent and the strip goes from smaller elements (left) to bigger elements (right). My goal is to make this strip interactive to mouse events.
Each element is some kind of polygonal image which is trimmed left and right and then exported as a png. It is then imported into a canvas.
The problem is that I can put them side by side but since they are not rectangles I need a way to calculate the offset made up by the transparent pixels on each side of each element to make them stick together correctly... I am using KineticJs to get a precise hitarea for each element... So maybe there is a way to do it automatically with kineticjs,or there is some kind of operation I could do using each image data?
My problem illustrated:
Any ideas?
Also I am doing this simply because I would prefer precise mouseOver bounding box on each item (rather than a simple rectangle) and would rather avoid the solution to calculate each offset manually... But maybe that's not worth it?!
Ok, so you have yourself a custom shape you want to use, here is a tutorial for that: http://www.html5canvastutorials.com/kineticjs/html5-canvas-kineticjs-shape-tutorial/ , the simplest thing you can do, and even that seems fairly long, is to calculate the bounding lines for that shape. (Two somewhat vertical lines, and two somewhat horizontal lines). Then you test if the right vertical line of shape one crosses with the left vertical line of shape two, if they do, then set the coordinates of the images to be the same coordinate.
http://www.mathopenref.com/coordintersection.html
line1 = ax + b ..... line2 = cx+d //see possible tests
if(...intersection test...){ // or just test if some coordinate is left of some other coordinate
shape2.setX(shape1.getX()+shape1.getWidth()); //account for image width, so they don't overlap
shape2.setY(shape1.getY()) // no need to account for height
}
UPDATE: This is a very rough solution to the workings of the problem. The next step would be to do more fine tuning dependent on each image.
http://jsfiddle.net/9jkr7/15/
If you want precise areas, use an image map. With some clever finagling and a blank image gif you should be able to have the background you want whenever you hover over any particular area of the image map (might require javascript).
The other option I can think of would be to use SVG itself or one of the many libraries in existance to build interactive vector graphics into your page.
You could also write a function that calculates the left most, top most, right most, and bottom most pixel by looking at all of the pixels in the image data. Here's a tutorial on that:
http://www.html5canvastutorials.com/advanced/html5-canvas-get-image-data-tutorial/

HTML5 Canvas animation issue

I am experimenting with HTML5 Canvases in an attempt to create some animation. My ultimate goal is to be able to animate a box to a particular location at whim, for now im just animating it across the screen. When I move it across the screen I geta black trail left behind, how do I clear this "dirty" section without removing the background grid?
A jsFiddle of the code is here
Two solutions
Redraw the background on the top of animation before moving it to the new location. This so called dirty sprite technique - faster - more complex.
Redraw the whole canvas between frames
If drawing the background is a complex operation just hold a prepared background buffered in another canvas for speed.
You have to clear what you've drawn if you don't want it to be visible. I assume you don't want to clear entire canvas to avoid redrawing grid and spending CPU cycles. You'll have to do this differently.
Possible solutions:
have two same onscreen canvases one over the other. Draw grid on canvas below and don't clear it. Clear part of top canvas and redraw on it.
have one on-screen and one off-screen canvas. Draw grid on off-screen canvas. Each time you animate, clear whole on-screen canvas, copy prepared grid from other one, and draw what you need over it.

How to use javascript to get circle on HTML canvas to move to point onmouseclick?

So if I draw a circle on HTML5 canvas, I want to make it so that the user can click somewhere in the canvas and the circle of a given radius will move gradually to the point mouseclicked on the canvas. I know jquery has the animate function, but since the canvas does not have a DOM, I am not sure how to accomplish this.
You will need to continuously repaint the canvas in a loop. A canvas is just a bunch of pixel data. If you want something to animate you have to draw each frame.
First clear the canvas to white (or any other color). Calculate the new position for the circle. Draw it. Wait for a short amount of time. Repeat.

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