The private TinyPNG alternative

Quantisation versus quality settings

The best-known PNG compressors work largely by reducing the number of distinct colours in an image and storing it as an indexed palette. On interface graphics, logos and flat illustration this is close to free — such images contain few colours to begin with — and it produces the dramatic reductions those tools are known for.

On a photograph the same technique is much less effective, because photographs contain thousands of colours that cannot be discarded without visible banding. That is why compression results vary so much by image type rather than by tool.

Targeting a size instead of a quality

Most compressors expose a quality slider, which answers the wrong question when you have a hard limit to meet. Being told an image is 'quality 80' does not tell you whether it fits under a two megabyte cap.

Searching for the quality that lands just under a target size answers the actual question, at the cost of encoding the image several times. Done in the browser that is only your own processing time, which is why the approach is practical here.

Changing the format usually beats tuning the quality

A JPEG squeezed hard develops blocking artefacts long before it approaches the size of the same image encoded as WebP at a comfortable quality. Format choice moves the whole curve; the quality slider only moves you along it.

The reason to stay with the original format is compatibility with wherever the file is going, which is a real constraint for email attachments and upload forms. Where you control the destination, converting is usually the larger win.

The batch and privacy trade

Upload-based compressors typically cap free use by file count or size per month, because each image consumes their bandwidth twice.

• Local compression has no monthly quota, because there is no bandwidth to consume.
• Your device does the encoding, so a large batch takes real time on an old machine.
• Nothing is transmitted, which matters for unreleased product shots and personal photos.
• Dedicated encoders can still edge out a browser on absolute ratio for a given format.