Convert BMP to PNG

How it works

The browser decodes the BMP, draws it to a canvas, and encodes the result as PNG, on our server.

The size difference is the whole point of the conversion. BMP files are large enough that the upload dominates the time — a 36 MB bitmap takes considerably longer to send over a typical connection than a browser takes to re-encode it. This is one of the cases where the local route is simply faster.

Why the saving is so large

BMP stores pixels and nothing else. Three bytes per pixel for 24-bit color, laid out row by row, with no attempt to notice that a thousand adjacent pixels are the same shade of gray. The size of the file is determined entirely by its dimensions.

PNG runs the same pixel data through Deflate, preceded by a filtering step that predicts each pixel from its neighbors and stores only the difference. On an interface screenshot, where huge runs of pixels really are identical, those differences are nearly all zero and compress to almost nothing. Hence the twentyfold reduction in the first example.

On a photograph the prediction rarely succeeds — adjacent pixels differ slightly almost everywhere — so the saving falls to roughly half. That is the whole story of lossless compression: it exploits repetition, and photographs have little.

Where BMP files still come from

The format is largely obsolete on the web but persists in a few places, which is why people still need this conversion.

Older Windows software. Paint saved BMP by default for years, and some in-house tools still do.

Scientific and industrial instruments. Microscope cameras, measurement devices and machine-vision systems often write BMP because it is trivial to implement and needs no library.

Scanners. Some drivers still offer it as the "no quality loss" option, which is true and also true of PNG at a fraction of the size.

Embedded systems. Anything that reads image data directly into a framebuffer benefits from a format with no decoding step, which is exactly what BMP was for.

In all four cases the reason is that BMP is simple to write, not that it is a good way to store a finished image. Converting is nearly always the right move once the file leaves the device that produced it.