RGBE image format
RGBE is an image format invented by Gregory Ward Larson. It stores pixels as one byte each for RGB (red, green, and blue) values with a one byte shared exponent. Thus it stores four bytes per pixel.
RGBE's biggest advantage is that it allows pixels to have the extended range and precision of floating point values. Often when images are generated from light simulations, the range of pixels values is much greater than will nicely fit into the standard 0..255 range of standard 24-bit image formats. As a result, the bright pixels are either clipped to 255 or end up losing all their precision in dimmer pixels. By using a shared exponent, the RGBE format gains some of the advantages of floating point values without the 12 bytes per pixel needed for single precision IEEE floating-point values, or 6 bytes in half precision (and which would cover smaller range). It can handle very bright pixels without loss of precision for darker ones.
A second variant of the format uses the XYZ color model with a shared exponent. The mime type and file extension is identical, thus applications reading this file format need to interpret the embedded information on the color model.
Image file formats
Image file formats are standardized means of organizing and storing digital images. Image files are composed of digital data in one of these formats that can be rasterized for use on a computer display or printer. An image file format may store data in uncompressed, compressed, or vector formats. Once rasterized, an image becomes a grid of pixels, each of which has a number of bits to designate its color equal to the color depth of the device displaying it.
Image file sizes
The size of raster image files is positively correlated with the resolution and images size (number of pixels) and the color depth (bits per pixel). Images can be compressed in various ways, however. A compression algorithm stores either an exact representation or an approximation of the original image in a smaller number of bytes that can be expanded back to its uncompressed form with a corresponding decompression algorithm. Images with the same number of pixels and color depth can have very different compressed file size. Considering exactly the same compression, number of pixels, and color depth for two images, different graphical complexity of the original images may also result in very different file sizes after compression due to the nature of compression algorithms. With some compression formats, images that are less complex may result in smaller compressed file sizes. This characteristic sometimes results in a smaller file size for some lossless formats than lossy formats. For example, graphically simple images (i.e. images with large continuous regions like line art or animation sequences) may be losslessly compressed into a GIF or PNG format and result in a smaller file size than a lossy JPEG format.
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