Compress PNG files and keep every pixel
There are two very different ways to compress PNG images, and most tools do not say which one they use. One reduces the number of colors in the picture, which can shrink a file dramatically but quietly changes it: gradients band, edges fringe, and transparency can go grainy. The other leaves the picture alone and only packs the file more tightly.
Lossless PNG Compressor does only the second. Each PNG is decoded and written again at maximum compression with an adaptive filter, so the image you get back is identical to the one you put in, down to the last pixel and alpha value. There is no quality slider because there is no quality to give up.
That honesty has a cost worth knowing: lossless savings depend on how carelessly the file was saved in the first place. A screenshot straight from a fast exporter often has room to spare; a PNG that has already been optimized may barely move.
How to reduce PNG file size in a batch
Click any file to see the method that applies to it and a bar comparing old and new size. Before a run the figure is labeled as an estimate, because the pixels are guaranteed but the saving is not.
- Click Add files, or drop PNG images onto the window. A folder dropped in brings its files with it.
- Read each row: the format, the current size and an estimate of what it should come down to.
- Choose where results go under Save: Beside the originals, Into a folder, or Replace the originals.
- Press Compress. Each row then shows the real before and after sizes, and the header totals what was saved.
PNG optimizer or color reducer: how to tell
If a tool promises a 70% smaller PNG, it is usually reducing the palette to 256 colors or fewer. That is fine for some web graphics and wrong for others, such as a design file you will edit again, a medical or scientific image, or a screenshot of fine text that has to stay crisp.
A true PNG optimizer only changes the compression of the data. You can check: open the original and the result side by side and zoom in on a gradient or a soft shadow. With this app they match.
Lossless image compression for TIFF and WebP too
The same queue accepts WebP, TIFF, BMP, Targa and QOI. WebP files are rewritten in WebP's lossless mode at its most thorough setting, and TIFF files are packed with LZW compression, which can make a large uncompressed scan much smaller with nothing changed.
JPEG is deliberately not accepted. A JPEG cannot be made smaller without discarding detail, and taking one would mean either pretending otherwise or doing the lossy thing behind your back.
