BMP to JPG converter

BMP is a way of writing an image where every point sits in the file separately, exactly as it is meant to appear on screen. The format was built for OS/2 and later moved into the Windows interface. It carries its own lossless RLE compression, although using it is optional and most programs leave it aside. Colour depth runs from 1 to 32 bits per pixel, and that last one also carries an alpha channel, meaning transparency.[1]

Microsoft describes the shape of such a file plainly: first comes a header with the file size and the offset where the image begins, then a second header with the dimensions and the number of bits per pixel, then an optional colour table, and at the end the raw array of points.[2]

JPG comes from a different idea. The name is the abbreviation of the Joint Photographic Experts Group, and the way of writing is set out in Recommendation ITU-T T.81, approved in September 1992.[3] The .jpg and .jpeg extensions mean exactly the same format; the shorter one caught on because systems before Windows 95 allowed only three characters after the dot.

The writing itself runs in several steps. The image is split into blocks of 8 by 8 pixels, a discrete cosine transform runs on each block, and the result passes through quantisation. Quantisation is where the loss of data happens. Separately, the resolution of the colour channels is reduced, which is called chroma subsampling.[4]

Every pixel in a finished JPG file is described by three colour components of eight bits each: red, green and blue. This format carries no alpha channel, and every browser version reads it.[5]

Drag your BMP files onto the Add files area, or click it and pick the images from your drive. The Set JPG quality slider takes values from 60 to 95 per cent and sits on 85 by default. Then press the Convert button.

You can download the finished images one at a time with the Download button on a queue row, or all together with Download all. The Clear all button empties the queue, and the Remove label on a row takes a single file out of it.

In the queue on the right, every file shows its weight before and after the conversion along with the difference in per cent. The same summary for the whole batch sits under the buttons, on the Saved line.

The browser itself does the writing. The image first goes onto the page canvas, the canvas element, and from there into a JPG file. This happens on your own device, so the file stays with you from the first click to the end of the work.

Under the converter sits a format switch reading BMP to JPG. Clicking either of the two formats opens a list, and picking from it takes you to the converter page for that pair. Every pair in this group is also gathered on the Format conversion page.

Because the weight of a BMP file follows from the dimensions of the image alone. In the 24-bit variant every pixel takes three bytes, and every row is padded with zeros up to a full four bytes; the Microsoft documentation gives a ready formula for it.[6] On top of that come 54 bytes of header. This arithmetic looks the same for every image, whatever the photo happens to show.

Let us follow it on a photo measuring 3000 by 2000 pixels. One row holds 3000 pixels at three bytes each, which makes 9000 bytes. At that width nothing needs padding. There are 2000 rows, so the pixels alone take 18,000,000 bytes. Together with the header the file weighs 18,000,054 bytes, which is 17.2 MB in the notation the summary inside the tool uses.

JPG approaches the same image differently. The encoder compares neighbouring points and leaves out the small differences between them. The quality slider sets that very threshold. The MDN documentation describes this value as a number between 0 and 1, handed to the encoder together with the image.[7] The higher you set the slider, the more detail stays in the file and the more the file weighs.

The Saved line then appears under the buttons, carrying the number of megabytes and the percentage the file has shed.