That’s a crucial distinction to understand for website performance.
Short Answer: Lossless compression reduces an image’s file size without any loss in quality, while lossy compression makes the file significantly smaller by permanently deleting some of the image’s data.
The choice between them comes down to a direct trade-off between perfect image quality and faster page load speeds.
To choose the right one, you need to understand how each method treats your image files. First, lossless compression works by reorganising the image data more efficiently without discarding any of it; think of it like a ZIP file where the original can be perfectly reconstructed. This makes it ideal for graphics with sharp lines and flat colours, like logos and icons, where formats like PNG excel. Second, lossy compression analyses the image and permanently removes data that the human eye is least likely to notice, resulting in a much larger reduction in file size at the cost of a minor drop in quality. This is the standard for web photographs, where using a JPEG format significantly improves your page speed. The most common mistake is saving large, complex photographs as lossless PNGs, which creates unnecessarily huge files that will cripple your site’s performance.
Cheers,
Jeff
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