BMP to JPG
Convert a BMP to JPG. An 8-bit palette BMP is down to 256 colours before JPEG sees anything, and run-length-encoded BMPs are refused.
BMP to JPG treats two kinds of BMP differently. An 8-bit one carries a 256-entry colour table, so the picture was reduced before JPEG saw it, and the smoothing lands on that reduced version. A run-length-encoded BMP is not converted at all: the reader takes uncompressed 8, 24 and 32-bit files and names the compression it stopped on.
Questions
What happens to a palette BMP?
An 8-bit palette BMP is already down to 256 colors before JPEG sees it, so the JPEG encoder works from whatever those 256 colors were rather than full continuous tone. A 24 or 32-bit BMP has no such limit going in.
Why would a BMP get refused?
Run-length-encoded BMPs are refused outright rather than decoded incorrectly, since this pipeline reads uncompressed bitmap data. An uncompressed BMP, the overwhelming majority of BMP files in practice, converts normally.
Which engine handles this?
A dedicated BMP decoder reads the file and jSquash encodes the JPEG through mozjpeg at the chosen quality, default 80. Both run in Node, on the command line, through MCP and in the browser.
What size should I expect?
Often much smaller, since BMP applies no compression at all and lossy JPEG compression cuts the file down considerably, often to a fraction of the original size.
What is the reverse page?
There is no dedicated jpg-to-bmp page. Convert image can write BMP from a JPG source directly, at the same uncompressed 32-bit format this reverse direction would use.