PNG to BMP Converter

Why convert PNG to BMP in the modern world

Converting PNG to BMP is an operation that at first glance seems like a step backward in technological development. PNG offers efficient lossless compression and full transparency support, while BMP stores data in virtually raw form, creating huge files. However, in certain professional areas, the simplicity and predictability of BMP make it an indispensable tool.

BMP (Bitmap Image File) is a format created by Microsoft in 1986 for the first versions of Windows. In nearly forty years of existence, it has barely changed, providing absolute backward compatibility. Any device or program capable of working with images is guaranteed to understand BMP. This universality is especially valuable when working with legacy software, industrial systems, and specialized equipment that hasn't been updated for decades.

PNG (Portable Network Graphics) appeared ten years later as a modern alternative to GIF. The format uses the DEFLATE algorithm for lossless compression and supports a full 8-bit alpha channel. However, this complexity becomes a problem in systems with limited resources or outdated software that lacks a PNG decoder.

Technical structure of BMP format

BMP file structure

BMP is one of the simplest raster formats, consisting of three sequential blocks:

File header (BITMAPFILEHEADER) — 14 bytes of fixed data:

• Signature «BM» (0x42 0x4D) — format identifier
• Total file size in bytes
• Two reserved fields (usually zeros)
• Offset from file start to pixel data

Information header (BITMAPINFOHEADER) — minimum 40 bytes:

• Header size (determines format version)
• Image width and height in pixels
• Number of color planes (always 1)
• Color depth: 1, 4, 8, 16, 24, or 32 bits
• Compression type (usually 0 — no compression)
• Pixel data size
• Horizontal and vertical resolution in pixels per meter
• Number of colors in palette
• Number of important colors

Pixel data — the actual image, written row by row from bottom to top (unlike most formats that scan top to bottom). Each row is aligned to 4-byte boundaries by adding padding bytes.

Data storage features in BMP

The main feature of BMP is the absence of compression in typical usage scenarios. Technically, the format supports RLE compression for 4-bit and 8-bit images, but in practice this is rarely used. Standard 24-bit BMP (True Color) writes each pixel with three bytes in BGR order (blue-green-red), which is the reverse order compared to the familiar RGB.

Formula for 24-bit BMP size without compression:

Size = 54 + ((Width × 3 + Padding) × Height)

where Padding = (4 - (Width × 3) mod 4) mod 4

Example: 1920×1080 pixel image

• Row: 1920 × 3 = 5760 bytes
• Padding: (4 - 5760 mod 4) mod 4 = 0 bytes
• Data: 5760 × 1080 = 6,220,800 bytes
• Headers: 54 bytes
• Total: about 5.93 MB

For comparison: the same PNG file with a photo would take 1-3 MB depending on content.

Comparison of PNG and BMP: key differences

Characteristics table

What happens to transparency during conversion

PNG supports a full 8-bit alpha channel with 256 levels of transparency. BMP can technically store an alpha channel in 32-bit mode (BGRA), but support for this feature in software is extremely unpredictable. Most applications ignore the 4th byte or interpret it incorrectly.

When converting PNG to BMP, transparent areas must be handled in one of these ways:

• Fill with white — standard approach, suitable for most tasks
• Fill with black — for images placed on dark backgrounds
• Fill with custom color — when the target background is known

Semi-transparent pixels (alpha 1 to 254) are blended with the fill color using the alpha compositing formula:

Result = Source × Alpha + Background × (1 - Alpha)

where Alpha is normalized from 0 to 1.

This means complex effects — blurred shadows, gradient transparency, glass overlays — are "flattened" onto the selected background during conversion, and restoring the original transparency is impossible.

Use cases for PNG to BMP conversion

Industrial equipment and CNC machines

A significant portion of industrial equipment runs on embedded software created in the DOS and early Windows era. CNC machines, laser engravers, plotters, embroidery machines, and other devices often accept graphics only in BMP format.

Reasons for this limitation:

• Simple decoding — BMP doesn't require complex decompression algorithms
• Minimal system requirements — decoder takes only a few kilobytes of code
• Predictable memory size — decoded image size is known in advance
• Reliability — simple format is less prone to errors

Converting PNG to BMP allows preparing modern images for loading into such equipment.

Legacy corporate software

Corporate environments often use software systems 15-25 years old: specialized ERP systems, medical software, document management systems, legacy applications. Updating such software involves huge costs and risks, so companies continue using proven solutions.

Many of these systems were developed in the Windows 95/98/XP era and support a limited set of graphics formats. BMP is practically the only format guaranteed to work everywhere.

Embedded systems and microcontrollers

Embedded systems developers often choose BMP due to the simplicity of working with the format:

• No library needed for PNG decoding (memory savings)
• Direct access to pixel data
• Ability to load image directly into video memory
• Predictable processing time

For displays in microcontroller projects (Arduino, ESP32, STM32), BMP is often the optimal format for transferring images from a computer.

File size comparison

Converting PNG to BMP almost always results in significant file size increase:

The largest increase is observed for screenshots and simple graphics, where PNG is especially effective due to large solid-color areas. For photos, the difference is smaller since PNG doesn't compress photographic content as efficiently.