Even though I can't visually see it on my current machine, I understand the issue you're facing and there are two ways to address it.
The root of the problem lies in pixel sampling. When an image is resized, there is a disparity between the screen pixels and the pixels in the image. Zooming out results in more pixels in the image than can be displayed, while zooming in causes one pixel to spread across multiple pixels.
The simplest solution is using nearest pixel sampling. The hardware determines where each device display pixel falls on the scaled image and selects the nearest pixel from the image.
Another common approach is bilinear interpolation. This method involves using the closest 4 pixels when zooming in and calculating the color by averaging the colors of neighboring pixels. When zooming out, it averages the pixels under the display pixel. While this method reduces flicker significantly, it may not eliminate it completely.
The canvas allows you to choose between nearest pixel sampling and the smoothing mode of the hardware, with the specific method depending on the hardware configuration.
To enable the smoothing method, set the imageSmoothingEnabled property of the 2D context to true.
ctx.imageSmoothingEnabled = true;
To disable it:
ctx.imageSmoothingEnabled = false;
By default, this option is usually set to "true" for the canvas, so toggling it may not immediately resolve your issue.
If you're using a PC, your graphics driver might come with a utility for adjusting rendering performance. These utilities are typically found in the Windows control panel and offer settings for optimizing performance or quality. If the performance setting surpasses the canvas smoothing option, it will prioritize speed over appearance. Switching to the best quality setting could potentially mitigate the flickering issue.
Some drivers provide granular control over individual graphics options, including pixel filtering preferences. If necessary, configure these settings to align with your preferences. It's possible to assign pixel filtering specifically to the browser application to ensure consistent rendering.
If all settings seem correct but flickering persists, alternative methods can be employed to minimize the problem.
Image blurring:
To counteract flickering during zoom-out actions, consider implementing a modified Mip-mapping technique. Create a slightly blurred copy of the image in memory and render that version when zoomed out. Transition between the blurred and original images subtly to prevent abrupt visual shifts. Utilize ctx.globalAlpha to fade between the images smoothly.
Mip mapping:
This strategy involves generating multiple copies of the image at decreasing resolutions. By strategically selecting which two images to display based on the zoom level, a seamless transition can be achieved using opacity adjustments.
Flickering artifacts may persist despite these efforts due to inherent limitations in scaling algorithms. Each method has its pros and cons, and for optimal quality, beginning with an image resolution twice that of the largest anticipated zoom level is advisable. Efficient sampling techniques are crucial for impeccable rendering, often necessitating GPU utilization through frameworks like WebGL.
Additional test for browser smoothing:
A diagnostic snippet has been included to assess whether browser smoothing settings are being overridden by the graphics driver on your system. One pattern features smoothing disabled, while the other uses enabled smoothing. If both patterns appear identical, external factors may be overpowering the browser's intended behavior.