Fundamental Differences Between PNG and JPG: A Selection Guide for Lossless and Lossy Compression
This article compares DEFLATE lossless encoding and DCT lossy encoding, covering file size, transparency support, and generational quality loss, to help designers and operators select the correct export format.
Cập nhật 2026-08-11
Core Differences in Encoding Principles
PNG uses DEFLATE lossless encoding. This algorithm matches repeated pixel sequences through the LZ77 sliding window, then compresses statistical redundancy with Huffman coding, and can completely restore the original pixel data after decoding.
JPG uses DCT (Discrete Cosine Transform) lossy encoding. This encoding divides the image into blocks for frequency transformation, discards high-frequency detail information that is insensitive to the human eye, and exchanges image quality loss for smaller storage space.
Comparison of File Size Differences for Different Content Types
Natural photographs contain a large number of gradient colors, DEFLATE compression has little redundancy, and the file size of PNG is usually 5 to 10 times that of JPG with the same resolution. After JPG discards high-frequency details, its file size compression advantage is obvious.
For UI screenshots and icons dominated by solid color blocks, DEFLATE can efficiently match repeated pixels. The file size advantage of PNG is reduced, and the size of PNG for some low-complexity screenshots is even smaller than that of medium-quality JPG.
| Content Type | PNG File Size Range | JPG File Size Range (Quality 75) |
|---|---|---|
| Natural photograph | 2MB-10MB | 300KB-1.5MB |
| UI interface screenshot | 200KB-800KB | 150KB-600KB |
| Solid color icon | 2KB-20KB | 10KB-50KB |
Differences in Transparency Channel and Generational Loss
PNG natively supports a complete 8-bit transparency channel, enabling 0 to 255 levels of semi-transparent gradient, and is compatible with all mainstream browsers and design software. JPG does not support transparency channels, and can only export completely opaque image content.
Repeated saving of PNG does not cause quality loss, only minor fluctuations in file size due to changes in compression parameters. JPG discards information again every time it is saved. After multiple saves, generational loss accumulates, and image blurring and color blocking gradually worsen.
- PNG supports complete Alpha semi-transparency channel, JPG does not support transparency
- PNG has no generational quality loss when saved repeatedly, JPG accumulates loss after multiple saves
- PNG can be used for source file storage that requires secondary editing
Selection Decision Tree by Content Type
When natural photography, portrait and landscape content is used for web display or new media distribution, choosing JPG can achieve a smaller file size with acceptable image quality to meet loading speed requirements.
For UI interface screenshots, line icons, vector-to-raster graphics, and design drafts with transparent elements, choosing PNG preserves clear edges and complete transparency, avoiding noise and blurring generated by JPG. For design drafts requiring print output, choosing PNG retains complete pixel information to meet printing accuracy requirements.
Câu hỏi thường gặp
Do I have to use PNG for transparent backgrounds?
PNG currently is the transparent format with the best compatibility for general scenarios, with full platform support for complete semi-transparency. WebP also supports transparency, but some older software is not compatible. Choosing PNG for regular export is more stable.
Can converting JPG to PNG recover image quality?
No. JPG has already discarded the original pixel information, converting it to PNG will only increase the file size and cannot restore the already lost details. It is recommended to export the original file directly to PNG.
Should I choose PNG or JPG for print output?
PNG is preferred for print output. PNG retains complete original pixels, and no additional loss will occur under the 300DPI accuracy requirement for printing. The compression loss of JPG will be amplified after printing, affecting the clarity of the finished product.