HDR to PGM Converter

HDR (also known as RGBE or Radiance HDR) is a high-dynamic-range image format created by Greg Ward Larson as part of the Radiance lighting simulation system, developed at Lawrence Berkeley National Laboratory starting in 1985 with the HDR format emerging around 1989. The format stores floating-point RGB pixel values using a compact 32-bit-per-pixel encoding called RGBE (Red, Green, Blue, Exponent): three 8-bit mantissa bytes share a single 8-bit exponent, representing luminance values across a range of roughly 76 orders of magnitude while keeping file sizes comparable to standard 24-bit images. HDR files begin with a text header containing rendering and exposure metadata, followed by the RGBE pixel data compressed with a scanline-oriented run-length encoding scheme. The format captures the full luminance range of real-world scenes — from deep shadows to direct sunlight — enabling physically accurate lighting calculations, tone mapping to different display conditions, and post-capture exposure adjustment without the clipping artifacts inherent in 8-bit formats. One advantage is the format's foundational role in HDR imaging: Radiance HDR pioneered the concept of storing real-world luminance values in image files, and the .hdr format became the standard for light probe images and environment maps used in image-based lighting across the 3D rendering industry. The format's compact encoding is another practical strength — the RGBE scheme provides far more dynamic range than 8-bit formats while using only 33% more storage per pixel, a favorable tradeoff that made HDR practical on storage-limited systems of the late 1980s. HDR files are supported by Photoshop, GIMP, ImageMagick, Blender, and all major 3D renderers.