When drawing on a canvas in firefox, if you attempt to draw at or move to a point beyond the bounds of the canvas, an error will be thrown (this is copied/pasted directly from Firebug):
An invalid or illegal string was specified" code: "12
Other browsers won't throw the error. This is for both the drawing methods (lineTo, arc, etc), as well as image drawing. It's easy enough to do a quick bounds check using the width and height of the canvas before drawing, but what about drawing parts of things?
For example, let's say you have a circle, defined by a center point and a radius, and drawn with an arc(). If you do a bounds check, the center could be off the canvas, while part of the circle should still technically be visible. Therefore, the circle will either be drawn completely or not at all: there's no way to make only part of the circle drawn if the center point is off the canvas.
Does anyone have a work around for this?
One that I thought of was using a large "world buffer" canvas that is then copied from to the display canvas. Another might be drawing each object in the center of a buffer canvas, then copying the newly drawn pixels to the display canvas in their proper position.
What have people done to work around this issue?
The canvas crops the image to the bounding rectangle of its view. You should have no problem drawing outside this bounding box. It will not show up on your drawing. Post a particular example to see if we can reproduce the error.
Related
Currently I have an image that needs to be manipulated so it matches the same scale, position, and rotation as a template.
The grey rectangle with a circle in the middle is the template.
The orange rectangle and circle represents the user's input. It needs to be rotated, scaled and aligned to it matches the grey one. I'm currently stumped on how to proceed. I've no code other than the following.
function align_image()
{
// clever transform alignment code here
}
Bad dog, no biscuit!
The process at of aligning the images would normally be done manual input and judged by eye. I'm hoping to automate this step and align the image to its respective size and position but leaving the comfort and safety of Photoshop DOM I'm not sure how to proceed or even if this is a trivial matter or one left best alone. The project is web based currently using javascript and three.js
So if anyone can give me some pointers I'd appreciated it.
I don't code javascript so I can only talk about the algorithm. Generally best tool for registration is to use feature matching methods (using sift, surf,...) but your image is not the kind that have strong features. Now if you're always dealing with rectangles and circles in your images, find the "edges" of the rectangle with Hough Transform, compute the angle of those edges (lines) then rotate the image with that angle in the opposite direction.
Then with the help of Hough Circle Detector, find the center of the circles in the middle of the images, calculate the distance between them, and move the target rectangle to the source's circle position. After the movement by comparing the radius of the circles, you can resize the target image to make it like the source rectangle.
All of these are conveniently doable with Opencv.
i'm a beginner in Javascript so please bear with me.
Basically I'm making a sandbox drawing facility for a website using Javascript. And this is done using the canvas. What I need to do is to be able to resize the canvas dynamically but at the same time keep everything on the canvas to scale.
I don't think this question has been asked before. It seems trivial but I currently have all my objects on the canvas defined in absolute coordinates. I also have mouse events to use to draw things. And when I want to enlarge the canvas (by doubling the size say). All the objects inside won't be enlarged properly to scale and the mouse coordinate system would be messed up too.
Only solution i can think of is add a scale factor to ALL my drawing parts, but this is very tricky with a lot of code. Is there a better way?
If you don't mind jaggies on your double-sized canvas drawings then you can simply use CSS to double-size your canvas. Then divide every incoming mouseEvent coordinate by 2.
If you don't want jaggies on your double-sized canvas then:
Double-size the canvas element: canvas.width*=2 and canvas.height*=2 This automatically erases all canvas content.
Scale up the canvas: context.scale(2,2)
Redraw all your drawing parts using the unchanged original coordinates. A happy note: you do not have to scale any of your drawing coordinates -- context.scale automatically does that for you.
Divide every incoming mouseEvent coordinate by 2.
I want to create a canvas that draws a background image but allows zooming and dragging as well. I came across this stackoverflow answer Zoom Canvas to Mouse Cursor. It's perfect except for one thing, it scales every image it draws because of the canvas scale factor.
I need to draw some markers on the image that don't scale when zooming in, kinda like those red arrows on google maps.
So I tried scaling the size of the image by the inverse of the zoom factor so it would be constant-sized and it worked but of course it doesn't stay at a specific spot on the image. Why does this happen and how should I fix it?
Draw 2 canvases on each other.The upper one(with a higher zindex) will be utilized for drawing the markers.The lower canvas(with a lower zindex) will be used to draw image on.You should perform all your manipulations(zoom and pan) on the lower canvas context keeping the upper one unchanged.
I would like to make a simple javascript library that involves rotating, translating, and scaling the canvas. However, when I rotate the canvas, since the center of rotation is (0, 0), half of the content gets deleted. I would like to know how to not have it deleted.
Translate to the center of the canvas first, rotate, and then translate back.
Note that inevitably (unless you also scale up the canvas) you will end up with some corners cut off.
I'm trying to draw a tiled background using Javascript on an HTML5 canvas, but it's not working because shapes that intersect the edges of the canvas don't wrap around to the other side. (Just to be clear: these are static shapes--no motion in time is involved.) How can I get objects interrupted by one side of the canvas to wrap around to the other side?
Basically I'm looking for the "wraparound" effect that many video games use--most famously Asteroids; I just want that effect for a static purpose here. This page seems to be an example that shows it is possible. Note how an asteroid, say, on the right edge of the screen (whether moving or not) continues over to the left edge. Or for that matter, an object in the corner is split between all four corners. Again, no motion is necessarily involved.
Anyone have any clues how I might be able to draw, say, a square or a line that wraps around the edges? Is there perhaps some sort of option for canvas or Javascript? My google searches using obvious keywords have come up empty.
Edit
To give a little more context, I'm basing my work off the example here: Canvas as Background Image. (Also linked from here: Use <canvas> as a CSS background.) Repeating the image is no problem. The problem is getting the truncated parts of shapes to wrap around to the other side.
I'm not sure how you have the tiles set-up, however, if they are all part of a single 'wrapper' slide which has it's own x,x at say 0,0, then you could actually just draw it twice, or generate a new slide as needed. Hopefully this code will better illustrate the concept.
// Here, the 'tilegroup' is the same size of the canvas
function renderbg() {
tiles.draw(tiles.posx, tiles.posy);
if(tiles.posx < 0)
tiles.draw(canvas.width + tiles.posx, tiles.posy);
if(tiles.posx > 0)
tiles.draw(-canvas.width + tiles.posx, tiles.posy);
}
So basically, the idea here is to draw the groupings of tiles twice. Once in it's actual position, and again to fill in the gap. You still need to calculate when the entire group leaves the canvas completely, and then reset it, but hopefully this leads you in the correct direction!
You could always create your tillable image in canvas, generate a toDataUrl(), and then assign that data url as a background to something and let CSS do the tiling.. just a thought.
Edit: If you're having trouble drawing a tillable image, you could create a 3*widthx3*width canvas, draw on it as regular (assuming you grab data from the center square of data as the final result), and then see if you can't draw from subsets of the canvas to itself. Looks like you'd have to use:
var myImageData = context.getImageData(left, top, width, height);
context.putImageData(myImageData, dx, dy);
(with appropriate measurements)
https://developer.mozilla.org/En/HTML/Canvas/Pixel_manipulation_with_canvas/
Edit II: The idea was that you'd have a canvas big enough that has a center area of interest, and buffer areas around it big enough to account for any of the shapes you may draw, like so:
XXX
XCX
XXX
You could draw the shapes once to this big canvas and then just blindly draw each of the areas X around that center area to the center area (and then clear those areas out for the next drawing). So, if K is the number of shapes instead of 4*K draws, you have K + 8 draws (and then 8 clears). Obviously the practical applicability of this depends on the number of shapes and overlapping concerns, although I bet it could be tweaked. Depending upon the complexity of your shapes it may make sense to draw a shape 4 times as you originally thought, or to draw to some buffer or buffer area and then draw it's pixel data 4 times or something. I'll admit, this is some idea that just popped into my head so I might be missing something.
Edit III: And really, you could be smart about it. If you know how a set of objects are going to overlap, you should only have to draw from the buffer once. Say you got a bunch of shapes in a row that only draw to the north overlapping region. All you should need to do is draw those shapes, and then draw the north overlapping region to the south side. The hairy regions would be the corners, but I don't think they really get hairy unless the shapes are large.... sigh.. at this point I probably need to quiet down and see if there's any existing implementations of what I speak out there because I'm not sure my writing off-the-cuff is helping anybody.