I have 2 canvases, one uses HTML attributes width and height to size it, the other uses CSS:
<canvas id="compteur1" width="300" height="300" onmousedown="compteurClick(this.id);"></canvas>
<canvas id="compteur2" style="width: 300px; height: 300px;" onmousedown="compteurClick(this.id);"></canvas>
Compteur1 displays like it should, but not compteur2. The content is drawn using JavaScript on a 300x300 canvas.
Why is there a display difference?
It seems that the width and height attributes determine the width or height of the canvas’s coordinate system, whereas the CSS properties just determine the size of the box in which it will be shown.
This is explained in the HTML specification:
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.
To set the width and height on a canvas, you may use:
canvasObject.setAttribute('width', '150');
canvasObject.setAttribute('height', '300');
For <canvas> elements, the CSS rules for width and height set the actual size of the canvas element that will be drawn to the page. On the other hand, the HTML attributes of width and height set the size of the coordinate system or 'grid' that the canvas API will use.
For example, consider this (jsfiddle):
var ctx = document.getElementById('canvas1').getContext('2d');
ctx.fillStyle = "red";
ctx.fillRect(10, 10, 30, 30);
var ctx2 = document.getElementById('canvas2').getContext('2d');
ctx2.fillStyle = "red";
ctx2.fillRect(10, 10, 30, 30);
canvas {
border: 1px solid black;
}
<canvas id="canvas1" style="width: 50px; height: 100px;" height="50" width="100"></canvas>
<canvas id="canvas2" style="width: 100px; height: 100px;" height="50" width="100"></canvas>
Both have had the same thing drawn on them relative to the internal coordinates of the canvas element. But in the second canvas, the red rectangle will be twice as wide because the canvas as a whole is being stretched across a bigger area by the CSS rules.
Note: If the CSS rules for width and/or height aren't specified then the browser will use the HTML attributes to size the element such that 1 unit of these values equals 1px on the page. If these attributes aren't specified then they will default to a width of 300 and a height of 150.
The canvas will be stretched if you set the width and height in your CSS. If you want to dynamically manipulate the dimension of the canvas you have to use JavaScript like so:
canvas = document.getElementById('canv');
canvas.setAttribute('width', '438');
canvas.setAttribute('height', '462');
The browser uses the css width and height, but the canvas element scales based on the canvas width and height. In javascript, read the css width and height and set the canvas width and height to that.
var myCanvas = $('#TheMainCanvas');
myCanvas[0].width = myCanvas.width();
myCanvas[0].height = myCanvas.height();
Shannimal correction
var el = $('#mycanvas');
el.attr('width', parseInt(el.css('width')))
el.attr('height', parseInt(el.css('height')))
Canvas renders image by buffer, so when you specify the width and height HTML attributes the buffer size and length changes, but when you use CSS, the buffer's size is unchanged. Making the image stretched.
Using HTML sizing.
Size of canvas is changed -> buffer size is changed -> rendered
Using CSS sizing
Size of canvas is changed -> rendered
Since the buffer length is kept unchanged, when the context renders the image,
the image is displayed in resized canvas (but rendered in unchanged buffer).
CSS sets the width and height of the canvas element so it affects the coordinate space leaving everything drawn skewed
Here's my way on how to set the width and height with Vanilla JavaScript
canvas.width = numberForWidth
canvas.height = numberForHeight
I believe CSS has much better machinery for specifying the size of the canvas and CSS must decide styling, not JavaScript or HTML. Having said that, setting width and height in HTML is important for working around the issue with canvas.
CSS has !important rule that allows to override other styling rules for the property, including those in HTML. Usually, its usage is frowned upon but here the use is a legitimate hack.
In Rust module for WebAssembly you can do the following:
fn update_buffer(canvas: &HtmlCanvasElement) {
canvas.set_width(canvas.client_width() as u32);
canvas.set_height(canvas.client_height() as u32);
}
//..
#[wasm_bindgen(start)]
pub fn start() -> Result<(), JsValue> {
// ...
let canvas: Rc<_> = document
.query_selector("canvas")
.unwrap()
.unwrap()
.dyn_into::<HtmlCanvasElement>()
.unwrap()
.into();
update_buffer(&canvas);
// ...
// create resizing handler for window
{
let on_resize = Closure::<dyn FnMut(_)>::new(move |_event: Event| {
let canvas = canvas.clone();
// ...
update_buffer(&canvas);
// ...
window.add_event_listener_with_callback("resize", on_resize.as_ref().unchecked_ref())?;
on_resize.forget();
}
}
There we update the canvas buffer once the WASM module is loaded and then whenever the window is resized. We do it by manually specifying width and height of canvas as values of clientWidth and clientHeight. Maybe there are better ways to update the buffer but I believe this solution is better than those suggested by #SamB, #CoderNaveed, #Anthony Gedeon, #Bluerain, #Ben Jackson, #Manolo, #XaviGuardia, #Russel Harkins, and #fermar because
The element is styled by CSS, not HTML.
Unlike elem.style.width & elem.style.height trick used by #Manolo or its JQuery equivalent used by #XaviGuardia, it will work for canvas whose size is specified by usage as flex or grid item.
Unlike the solution by #Russel Harkings, this also handles resizing. Though I like his answer because it is really clean and easy.
WASM is the future! Haha :D
P.S. there's a ton of .unwrap() because Rust explicitly handles possible failures.
P.P.S.
{
let on_resize = Closure::<dyn FnMut(_)>::new(move |_event: Event| {
let canvas = canvas.clone();
// ...
update_buffer(&canvas);
// ...
window.add_event_listener_with_callback("resize", on_resize.as_ref().unchecked_ref())?;
on_resize.forget();
}
can be done much cleaner with better libraries. E.g.
add_resize_handler(&window, move |e: ResizeEvent| {
let canvas = canvas.clone();
// ...
update_buffer(&canvas);
})
If you want a dynamic behaviour based on, e.g. CSS media queries, don't use canvas width and height attributes. Use CSS rules and then, before getting the canvas rendering context, assign to width and height attributes the CSS width and height styles:
var elem = document.getElementById("mycanvas");
elem.width = elem.style.width;
elem.height = elem.style.height;
var ctx1 = elem.getContext("2d");
...
I am using a GridView to create a gallery of images that I display from a certain folder on my machine. The images are of different width*height ratios. Few are portrait images where the height of the image is like 3 times more than width. Some image are landscape like where the width is approx 3 times of height. And some images are squared.
The issue with my gallery is I have a square for each Image component of my GridView as you can see below. This makes the portrait & landscape images get incorrectly stretched as each block in the Grid is a square. Thus causing the aspect ratio of the image to get skewed up.
The Question:
How can I ensure that I display both portrait & landscape images into my square without skewing the aspect ratio? I am looking at putting some black/grey background on parts of the square which go blank on a portrait or landscape image? But there might be other tricks.
The code:
import QtQuick 2.5
import Qt.labs.folderlistmodel 2.1
import QtQuick.Controls 1.1
import QtQml.Models 2.1
GridView {
cellHeight: height/2
cellWidth: width/2
clip: true
FolderListModel {
id: dir_model
folder: "/path/to/images/folder"
nameFilters: ["*.png"]
}
Component {
id: file_delegate
Image {
width: (parent.width)/2
height: (parent.width)/2
source: fileURL
}
}
model: dir_model
delegate: file_delegate
}
Just set Image fillMode property to Image.PreserveAspectFit
As the doc says:
the image is scaled uniformly to fit without cropping
The goal: Have a scroll view that displays an array of uiimageviews (photos) that you can horizontally scroll through them
How I understand to do this: Make the frame (CGRect) of each uiimageview the height and width of the scroll view, the y value to 0 on each, and set the first imgViews x value to 0. For every imgView after that, add the width of the scrollview to the x value. In theory, this would line the imgViews (Photos) up next to each other horizontally and not allow for any vertical scrolling or zooming, purely a horizontal photo viewer.
The storyboard setup: I am creating my scrollview in a xib file (It’s a custom uiCollectionViewCell), with these constraints:
— Top space to cell (0)
— Trailing space to cell (0)
— Leading space to cell (0)
— Height of 400
— Bottom space to a view (0)
—— (See Below for img)
Laying out the UIImgViews:
func layoutScrollView() {
for (index, img) in currentImages.enumerate() {
let imgView = UIImageView(frame: CGRect(x: CGFloat(index) * scrollView.bounds.width, y: CGFloat(0), width: scrollView.bounds.width, height: scrollView.bounds.height))
imgView.contentMode = .ScaleAspectFill
imgView.image = img
scrollView.addSubview(imgView)
scrollView.contentSize = CGSize(width: imgView.frame.width * CGFloat(index), height: scrollView.bounds.height)
scrollView.setNeedsLayout()
}
}
My suspicion: I suspect the issue is stemming from the auto layout constraints i’ve specified, but (considering Im asking a SO question) not sure
If there is a better way to do this (really the correct way) please let me know! I have been trying to wrap my head around this for a few days now.
I appreciate all responses! Thanks for reading
EDIT #1
I tried paulvs approach of setting setNeedsLayout & layoutIfNeeded before the "for" loop, and still no luck. Here is (out of three images selected) the second photo displaying. It seems that both the first and second photos are way longer than the content view and that would move the middle view over (Squished).
Your code looks fine except for a few details (that may be causing the problem):
Add:
view.setNeedsLayout()
view.layoutIfNeeded()
before accessing the scrollView's frame (a good place would be before the for-loop).
This is because when using Autolayout, if you access a view's frame before the layout engine has performed a pass, you will get incorrect frames sizes/positions.
Remove these lines from inside the for-loop:
scrollView.contentSize = CGSize(width: imgView.frame.width * CGFloat(index), height: scrollView.bounds.height)
scrollView.setNeedsLayout()
and place this line after (outside) the for loop:
scrollView.contentSize = CGSize(width: imgView.frame.width * CGFloat(currentImages.count), height: scrollView.bounds.height)
I am currently working on a project where I have a scroll view and four different screens. I have used a simple UIImage(named:"")!, code to put the frames of images into my imageView and animate the image.
I am having a problem where when I slide to my next screen the animation has already been completed. I do not know how to make it so that when I slide to the next screen the animation will then begin.
Here is the code for the animal animation
imageView.animationDuration = 1
imageView.animationRepeatCount = 1
imageView.animationImages =
[
UIImage(named:"Deer_00014")!,
UIImage(named:"Deer_00015")!,
UIImage(named:"Deer_00016")!,
UIImage(named:"Deer_00017")!,
UIImage(named:"Deer_00018")!,
UIImage(named:"Deer_00019")!,
UIImage(named:"Deer_00020")!,
UIImage(named:"Deer_00021")!,
UIImage(named:"Deer_00022")!,
UIImage(named:"Deer_00023")!,
UIImage(named:"Deer_00024")!,
UIImage(named:"Deer_00025")!,
UIImage(named:"Deer_00026")!,
UIImage(named:"Deer_00027")!,
UIImage(named:"Deer_00028")!,
UIImage(named:"Deer_00029")!,
UIImage(named:"Deer_00030")!,
UIImage(named:"Deer_00031")!,
UIImage(named:"Deer_00032")!,
UIImage(named:"Deer_00033")!,
UIImage(named:"Deer_00034")!,
UIImage(named:"Deer_00035")!,
]
self.imageView.image = UIImage(named: "Deer_00035")
imageView.startAnimating()
Just remove this line
self.imageView.image = UIImage(named: "Deer_00035")
Basically here you are setting the image to the ímageview' overwriting animation.
Here's a related image:
I want to achieve something like what's pictured on the right side of my image. But I also have a parent container that has a background image of its own, instead of a solid color.
Any advice?
EDIT: Forgot to add, cross-browser compatibility is important. (Or atleast Firefox).
I can only think of one pure CSS solution and it is simply insane.
Let's say your image has a width of 100px. You'll have to create a div that's 100px wide and give it 100 children that are each 1px wide, that each have the same background (positioned accordingly) and that each have an opacity from 0 (the first child) to .99 (the last child).
Personally, I think it's crazy and I'd never use this method.
Rory O'Kane came with a nice and clean solution and I also have another idea which involves JavaScript.
Basically, the idea is that you use a canvas element (support), draw your image on it, loop through its pixels and adjust the alpha for each.
demo
(scroll down to see the result)
Relevant HTML:
<div class='parent'>
<canvas id='c' width='575' height='431'></canvas>
</div>
Relevant CSS (setting the background image on the parent)
.parent {
background: url(parent-background.jpg);
}
JavaScript:
window.onload = function() {
var c = document.getElementById('c'),
ctxt = c.getContext('2d'),
img = new Image();
img.onload = function() {
ctxt.drawImage(img, 0, 0);
var imageData = ctxt.getImageData(0, 0, 575, 431);
for(var i = 0, n = imageData.data.length; i < n; i += 4) {
imageData.data[i + 3] = 255*((i/4)%575)/575;
}
ctxt.putImageData(imageData, 0, 0);
};
/* images drawn onto the canvas must be hosted on the same web server
with the same domain as the code executing it */
/* or they can be encoded like in the demo */
img.src = 'image-drawn-on-canvas.jpg';
};
check these out maybe helpful
DEMO 1
DEMO 2
Ignoring possible CSS-only methods, you can make the image a PNG with the transparent gradient built in to the image’s alpha channel. All browsers support PNG transparency, except for IE 6 and below. Here’s what your sample image would look like as a PNG with a transparent gradient (try putting this image against other backgrounds):
If the images are user-submitted so you can’t add the gradient ahead of time, you could create and store a gradient-added version of each image at the time that the user uploads them.
CSS only method:
https://gist.github.com/3750808