I use jcrop to provide users with a friendly way of uploading their images via ajax. Obviously these images have some constraint like width and height which is where jcrop comes into play. So for the sake of brevity what im doing is as follows:
input file select via javascript file api loads the image into a img tag. Jcrop works with this image tag and renders the result onto a html canvas.
Now this is the dodgy part. The canvas image is always blurry...
for arguments sake the canvas is set to 400x200 which is the crop size.
If the canvas width and height is set via CSS it results in a blurry image result. TO get around this I had to set the width and height via html attributes. Now I have a wonderful cropping solution that can save images via AJAX. Crisp and Clear:)
<canvas id="preview" width="400" height="200"></canvas>
According to HTML Standard, if width or height attribute is missing, then the default value must be used instead, which is 300 for width and 150 for height. And these two attributes will determine the canvas's width and height, while the css properties merely determine the size of the box in which it will be shown.
The canvas element has two attributes to control the size of the element's bitmap: width and height. These attributes, when specified, must have values that are valid non-negative integers. The rules for parsing non-negative integers must be used to obtain their numeric values. If an attribute is missing, or if parsing its value returns an error, then the default value must be used instead. The width attribute defaults to 300, and the height attribute defaults to 150.
The intrinsic dimensions of the canvas element when it represents embedded content are equal to the dimensions of the element's bitmap.
The user agent must use a square pixel density consisting of one pixel of image data per coordinate space unit for the bitmaps of a canvas and its rendering contexts.
A canvas element can be sized arbitrarily by a style sheet, its bitmap is then subject to the 'object-fit' CSS property.
You can refer to the following post for more details:
Canvas is stretched when using CSS but normal with "width" / "height" properties
This is really just an extension of the answer above. I too encountered the problem of CANVAS images resized using Javascript/CSS becoming fuzzy and blurry, because my application first used HTML to create a few DIVs, one of which held a CANVAS control, then the ONLOAD event called a Javascript routine to get the screen size, optimise the sizes and positions of all the DIVs and the CANVAS accordingly, and finally draw on the CANVAS. I did it that way as I wanted the CANVAS to always be as big possible for whatever device it was viewed on.
My solution is to use Javascript to dynamically draw the CANVAS control too, i.e. for the DIV that contains the CANVAS simply include...
var CanvasWidth=screen.availWidth-30;
var CanvasHeight=screen.availHeight-190;
//The 30 and 90 above are arbitrary figures to allow for other DIVS on the page
var o=window.document.getElementById("IdOfDivContainingCanvas");
o.innerHTML="<canvas id='myCanvas' width='"+CanvasWidth+"' height='+CanvasHeight+'></canvas>";
...so that the size of the CANVAS is effectively dynamic; Created with it's size specified using HTML attributes, just before executing the Javascript statements that actually draw on the CANVAS.
Related
I have a canvas element of size 1024x768 where I draw a very large image, let's just say with width >> 1024 and height >> 768. The image can be scrolled with normal scrollbars to be seen entirely.
Users can click onto the canvas to add vertices to a polygon: each time a vertex is added, a line from the new vertex to the mouse cursor is drawn.
Each mouse movement onto the canvas updates the canvas in order to redraw such line from the vertex to the new mouse cursor position.
Being the image VERY large, redrawing is very slow, so I'd like to just redraw the currently visible chunk of image or, even better, the "sub-chunk" that is affected by this line.
The problem is: how do I detect the current topLeft and bottomRight of the visible part of the context according to how much the user scrolled the image? Basically I'd like to add support for a custom viewport, but I can't seem to exploit the scolling information to determine the boundingBox of the chunk to crop from the original image.
I actually have a canvasDiv container, which is the real scrollable element, not the canvas itself.
This means your canvas is the full size and you're using the container div as a sort of a "frame". That's really bad as canvas pixels are quite expensive, especially if you're not even using them (they're out of the viewport).
The best approach here is to set the canvas size to the container size and use the drawImage function to control the scrolling. The original image would be places either on an offscreen canvas or an image element. I can guarantee you a much better performance this way.
You'll lose the scrollbars, that's the downside.
I'm assuming you create the scrollbars by having a larger canvas element contained inside a smaller div.
Instead, you can mimic this setup by:
Having a canvas the size of your container div.
Add 2 input type=range controls to act as vertical & horizontal scrollbars (or grab a jqueryUI scrollbar plugin if you want to be fancy).
Then you can use the clipping version of context.drawImage to draw only the portion of the image specified by the range controls. The clipping version of drawImage looks like this: drawImage(myLargeImage, clipX,clipY,clipWidth,clipHeight, 0,0,canvas.width,canvas.height)
As the title says, i want to add background pattern to a png image(not the whole canvas). I tried various guides but they are all about adding background pattern to whole canvas by either using background-image css property or by using canvas's createPattern method.
This is what i'm creating.
As you can see, there are various png images on the canvas(left arm, right arm, body, etc.) and they are all customizable. Now i want to add repeat-pattern to those png images. Those repeat pattern would be an image itself.
How to do that via svg, fabric or any other method?
A really simple way is to create a FOR loop to draw your PNG images at the size you need.
Use the canvas width/height values,your image size and your preferred image distance to determine
the number of iterations your loop needs to run.
On each iteration increment a distance variable a certain value in
order to blit your image at an increasing distance on the x/y axis.
When the loop ends you will have a pattern.
I have a large png image (15000x6000) that I load into chrome. I use the image for data analysis (convolutions etc).
I created a smaller version for viewing (coffeescript) via CSS style:
img.setAttribute 'style',
"position:absolute;top:0;left:0;width:#{w};height:#{h};z-index:#{z}"
where w, h are the width/height of the smaller image (z is used to create an ordered stack of elements)
This works, but seems to actually reduce the original image size too, rather than simply scaling the page display size! I.e. when I look at the original image width, it is reduced.
I get around this by creating a second, clone image of the original. I do this by setting the clone width/height to the smaller value, and then setting the img.src to the original src. This seems to work.
Is this the expected behavior? Do "thumbnails" and other image stunts actually decrease the initial image size? Profiling doesn't help .. chrome has removed its Native Memory functionality, sigh.
Do you have any idea how it could be possible not to scale a HTML canvas element while scaling the whole website (ctrl+"+" in most browsers) but to make its dimensions bigger? I have got an application where you can view large images, zoom and pan them in the canvas element. Now I think it would be cool to scale the website with ctrl+"+" to have more space for viewing the image. As it is by default the canvas scales, too and you gain nothing.
You can use technique described in this article http://novemberborn.net/2007/12/javascriptpage-zoom-ff3-128.
The main idea is to place any two elements and set for the first element css value in pixels like top\left\width etc and for the second element percentage value. When you scale the page the percentage value stays unchanged, but the value in pixels changes depending on zoom factor.
Based on these changes you can calculate the scale factor and multiple it with canvas dimensions to scale it.
How to calculate scale factor you can find in the demo link from article in the script block.
Something like Zoomooz.js could work - http://janne.aukia.com/zoomooz/
Is it possible to show any part of image in img tag (with pixels) via JavaScript?
I would have a prepared big image (e.g. 32x320 pixels) and defined starting position (X,Y , e.g. 0,32) and width/height (e.g. 32,32), and would want the script to show second (32x32 pixel) part of main image.
You could use CSS properties for this and change them via JS. Set the image as a background for an element with your desired size and adjust its position with background-position so that the correct part of it is visible. Some people call it CSS sprites.
For the sake of giving you multiple options, you could always use an HTML5 canvas and redraw the image as necessary. You can find a nice tutorial about how to do this here : https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Canvas_tutorial/Using_images
The CSS sprite method would be preferable nonetheless as IE doesn't support canvas yet.