get visible part of background image of element in javascript

To get the visible part of the background image of element, we can use the getComputedStyle method to get the background-image, background-position and background-size properties of the element. Then, we can calculate the visible background image based on the element's offset dimensions and position.

Here's an example code snippet:

index.tsx
// get the element
const element = document.getElementById("example");

// get computed styles for the element
const styles = window.getComputedStyle(element);
const bgImage = styles.backgroundImage;
const bgPosition = styles.backgroundPosition;
const bgSize = styles.backgroundSize;

// calculate the visible image
const img = new Image();
img.src = bgImage.slice(4, bgImage.length - 1);
img.onload = function() {
  const positionParts = bgPosition.split(" ");
  const sizeParts = bgSize.split(" ");

  const bgX = parseInt(positionParts[0]) || 0;
  const bgY = parseInt(positionParts[1]) || 0;
  const bgWidth = parseInt(sizeParts[0]) || img.width;
  const bgHeight = parseInt(sizeParts[1]) || img.height;

  const visibleX = Math.max(0, -bgX);
  const visibleY = Math.max(0, -bgY);
  const visibleWidth = Math.min(bgWidth, element.offsetWidth - bgX);
  const visibleHeight = Math.min(bgHeight, element.offsetHeight - bgY);

  console.log(`Visible portion of background image: ${visibleX}, ${visibleY}, ${visibleWidth}, ${visibleHeight}`);
};
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In this code, we first get the computed styles for the element using getComputedStyle. We then parse the background-image, background-position, and background-size properties to get the necessary background image properties.

Next, we create a new Image object and set its src to the URL of the background image. When this image has finished loading, we calculate the visible part of the image using the background-position, background-size, and element offset dimensions.

We calculate the visible portion of the image by first calculating the offset of the image using the background-position property. If the background-position is not specified, we use 0 as the default offset. We then calculate the size of the image using the background-size property. If the background-size is not specified, we use the width and height of the image as defaults.

Finally, we calculate the visible portion of the image by taking the maximum of 0 and the negative offset values (to ensure we start at the visible portion of the image), and the minimum of the size of the image and the difference between the element's offset dimensions and the offset of the image. We log the visible portion of the image to the console.