"Displaying <canvas> at its true resolution even when zoomed in, thanks to Retina technology

Question

"Displaying <canvas> at its true resolution even when zoomed in, thanks to Retina technology

I am currently working with a <canvas> element to create vector graphics and I want to ensure that it displays in the highest quality possible while using minimal resources across various viewing conditions. Specifically, I aim to achieve a 1:1 mapping between canvas pixels and device pixels to avoid blurry images and time-consuming pixel scaling operations.

I have come across some information about "Retina" displays and how the devicePixelRatio can help achieve this, but most sources assume viewing at 100% zoom level. Additionally, discussions about detecting zoom levels seem complex and imprecise, making it challenging to maintain the desired 1:1 ratio accurately.

Here are some questions that arise from this practical and theoretical perspective:

In modern browsers (released within the past 6 months), what is the best approach to achieve a 1:1 canvas without compromising on quality? Do I really need to detect both the device pixel ratio and zoom levels, accounting for changes in zoom as well?
If resource consumption is not an issue, would a 2:1 canvas resolution be beneficial (native resolution on "Retina" displays and up to 200% zoom compatibility elsewhere)? Is downscaling preferable to upscaling in such scenarios?
How do browser vendors address these concerns? Are there ongoing developments to provide better solutions, or is zoom level management considered a trivial matter for web developers? [Partly answered here: Many acknowledge its importance, but debates exist over adapting devicePixelRatio with zoom levels or introducing a new property for user-specified zoom detection and DPI differentiation.]
What percentage of internet users browse at non-100% zoom levels, especially those with high DPI displays? Are there any statistics available on this behavior? [Addressed here: Approximately 10% of Gmail users utilize non-100% zoom settings.]

css html canvas zooming retina-display

Answer 1

Answer №1

My research on Chrome and Internet Explorer revealed the subtle yet crucial differences between

canvas.width, and
canvas.style.width

It's imperative to understand that these two properties are not interchangeable. To manipulate them effectively, caution is required due to different units of measurement - some in physical pixels and others in logical pixels (based on a 96 dpi zoom).

In my scenario, I needed the Canvas to occupy the entire window.

The first step was to determine the size of the window in logical pixels (referred to as "CSS pixels"):

// Obtain size in CSS pixels (e.g., 96 dpi)
var styleWidth = window.innerWidth;   // Example: 420. Not document.body.clientWidth
var styleHeight = window.innerHeight; // Example: 224. Not document.body.clientHeight

These values represent logical pixels. When the browser zooms in, the number of logical pixels decreases:

Zoom  window.innerWidth x windows.innerHeight
====  ======================================
100%  1680 x  859
200%   840 x  448
400%   420 x  224
 75%  2240 x 1193

To find the actual window size, accounting for zoom level adjustments, we need a function like GetZoomLevel():

function GetZoomLevel(): number
{
}

Utilizing the GetZoomLevel() function, we can calculate the true window size in physical pixels. Thus, setting the width and height of the canvas to match the window size in physical pixels:

// Set canvas resolution to physical pixels
var newZoomLevel = GetZoomLevel();
myCanvas.width = styleWidth * newZoomLevel;   
myCanvas.height = styleHeight * newZoomLevel;

An essential point to note is that the canvas will render based on its set pixel dimensions. Additionally, CSS can later resize or distort the canvas:

Blurring when enlarged by CSS
Discarding pixel data if shrunk by CSS

To ensure the canvas occupies the entire client area window visually while maintaining full real pixel resolution internally, use CSS length Strings:

myCanvas.style.width = styleWidth + "px";   
myCanvas.style.height = styleHeight + "px";

GetZoomLevel()

Accessing the missing piece, GetZoomLevel, unfortunately, only IE provides this information:

function GetZoomLevel(): number {
    /*
        Details regarding DPI adjustment...
    */


    var zoomLevel: number;

    // Check if the browser supports the necessary API
    if (screen && screen.deviceXDPI && screen.logicalXDPI)
    { 
        //IE6 and above
        zoomLevel = (screen.deviceYDPI / screen.logicalYDPI);
    else
    {
        //Chrome (referenced from source)
        zoomLevel = window.outerWidth / window.innerWidth;
    }

    return zoomLevel;
}

No browsers except IE provide insights into:

Device DPI
Logical DPI

The complete TypeScript code snippet:

  
    Your modified text goes here....

Adjust your drawing code to account for the user's current zoom level to avoid incorrect measurements. For instance, drawing a box positioned at (50,50) shouldn't be drawn as 32px but rather as 128px at (200,200) with a zoom factor of 4.

Note: Any code shared is in the public domain. No attribution needed.

Answer 2

My research on Chrome and Internet Explorer revealed the subtle yet crucial differences between

canvas.width, and
canvas.style.width

It's imperative to understand that these two properties are not interchangeable. To manipulate them effectively, caution is required due to different units of measurement - some in physical pixels and others in logical pixels (based on a 96 dpi zoom).

In my scenario, I needed the Canvas to occupy the entire window.

The first step was to determine the size of the window in logical pixels (referred to as "CSS pixels"):

// Obtain size in CSS pixels (e.g., 96 dpi)
var styleWidth = window.innerWidth;   // Example: 420. Not document.body.clientWidth
var styleHeight = window.innerHeight; // Example: 224. Not document.body.clientHeight

These values represent logical pixels. When the browser zooms in, the number of logical pixels decreases:

Zoom  window.innerWidth x windows.innerHeight
====  ======================================
100%  1680 x  859
200%   840 x  448
400%   420 x  224
 75%  2240 x 1193

To find the actual window size, accounting for zoom level adjustments, we need a function like GetZoomLevel():

function GetZoomLevel(): number
{
}

Utilizing the GetZoomLevel() function, we can calculate the true window size in physical pixels. Thus, setting the width and height of the canvas to match the window size in physical pixels:

// Set canvas resolution to physical pixels
var newZoomLevel = GetZoomLevel();
myCanvas.width = styleWidth * newZoomLevel;   
myCanvas.height = styleHeight * newZoomLevel;

An essential point to note is that the canvas will render based on its set pixel dimensions. Additionally, CSS can later resize or distort the canvas:

Blurring when enlarged by CSS
Discarding pixel data if shrunk by CSS

To ensure the canvas occupies the entire client area window visually while maintaining full real pixel resolution internally, use CSS length Strings:

myCanvas.style.width = styleWidth + "px";   
myCanvas.style.height = styleHeight + "px";

GetZoomLevel()

Accessing the missing piece, GetZoomLevel, unfortunately, only IE provides this information:

function GetZoomLevel(): number {
    /*
        Details regarding DPI adjustment...
    */


    var zoomLevel: number;

    // Check if the browser supports the necessary API
    if (screen && screen.deviceXDPI && screen.logicalXDPI)
    { 
        //IE6 and above
        zoomLevel = (screen.deviceYDPI / screen.logicalYDPI);
    else
    {
        //Chrome (referenced from source)
        zoomLevel = window.outerWidth / window.innerWidth;
    }

    return zoomLevel;
}

No browsers except IE provide insights into:

Device DPI
Logical DPI

The complete TypeScript code snippet:

  
    Your modified text goes here....

Adjust your drawing code to account for the user's current zoom level to avoid incorrect measurements. For instance, drawing a box positioned at (50,50) shouldn't be drawn as 32px but rather as 128px at (200,200) with a zoom factor of 4.

Note: Any code shared is in the public domain. No attribution needed.

Answer 3

Answer №2

Detecting zoom level on modern browser versions of Firefox and Chrome can be a challenge:

Firefox 18+ throws a curveball by changing the devicePixelRatio value when manually zooming (cmd/ctrl +/-), making it tricky to determine whether the browser is in zoom mode or if it's on a retina device, despite the term DEVICE.

In Chrome 27 (using WebKit and Blink), webkitTextSizeAdjust was deprecated in desktop versions. This used to be a reliable method for detecting zoom in desktop Chrome. There are other methods available, but they may not cover all scenarios - like using SVG, which doesn't work in iFrames, or using window.inner/outerWidth, which is affected by sidebars or open DevTools.

Source

Answer 4

Detecting zoom level on modern browser versions of Firefox and Chrome can be a challenge:

Firefox 18+ throws a curveball by changing the devicePixelRatio value when manually zooming (cmd/ctrl +/-), making it tricky to determine whether the browser is in zoom mode or if it's on a retina device, despite the term DEVICE.

In Chrome 27 (using WebKit and Blink), webkitTextSizeAdjust was deprecated in desktop versions. This used to be a reliable method for detecting zoom in desktop Chrome. There are other methods available, but they may not cover all scenarios - like using SVG, which doesn't work in iFrames, or using window.inner/outerWidth, which is affected by sidebars or open DevTools.

Source

"Displaying <canvas> at its true resolution even when zoomed in, thanks to Retina technology

Answer №1

GetZoomLevel()

Answer №2

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