SPU File – What is .spu file and how to open it?

Definition and Purpose

An SPU file, short for “Spectrum 512 Image,” is a raster image format specifically designed for use with the Atari ST series of personal computers. It was introduced in the early 1980s as a standard image format for the ST’s GEM (Graphical Environment Manager) graphical user interface.

SPU files are characterized by their ability to store both color and black-and-white images in a compact and efficient format. They utilize a palette-based system, where a limited number of colors (typically 256 or less) are defined for the image. This allows for efficient storage of color images, as only the color references need to be stored, rather than the entire RGB values.

Technical Details

SPU files are stored in a simple binary format that consists of a header section and the image data. The header section contains information about the image’s dimensions, number of colors, and color palette. The image data is stored in a raster format, where each pixel is represented by a single byte.

The color palette is defined using a color lookup table (CLUT). Each entry in the CLUT consists of a 16-bit value that specifies the red, green, and blue components of the corresponding color. The CLUT can be either monochrome (black-and-white) or color, with a maximum of 256 colors.

SPU files can be compressed using the RLE (run-length encoding) algorithm to reduce their file size. RLE works by identifying and encoding consecutive runs of identical pixels, which are stored as a single byte followed by the number of repetitions. This compression technique can significantly reduce the size of SPU files, especially for images with large areas of solid color.

Method 1: Atari 8-bit Emulators

SPU files are primarily associated with Atari 8-bit computers, such as the Atari 400, 800, and XL series. To open and view SPU files, you can use Atari 8-bit emulators that can emulate the original hardware and run the image files. Popular emulators include Stella, Hatari, and Altirra. Once the emulator is installed, you can simply load the SPU file into the emulator’s file browser and it will be displayed on the screen.

Method 2: Image Viewers

Some image viewers and graphics editors also support the SPU file format. For instance, XnView, IrfanView, and GIMP are capable of displaying SPU images. However, it’s important to note that these viewers may not provide the full functionality of an Atari 8-bit emulator and may not accurately represent all aspects of the image. Nevertheless, they offer a convenient option for quickly viewing and converting SPU files into other image formats.

Format and Compatibility

The SPU (Spectrum 512 Image) file format is designed specifically for storing raster graphics on Atari’s Spectrum 512 home computer. It is a versatile format capable of handling both monochrome and color images, with support for resolutions ranging from 160×200 to 640×480 pixels. Additionally, SPU files can store various color palettes, allowing for customized image presentations. They offer compatibility with various software applications and emulators, enabling users to view and edit images on both original Spectrum 512 hardware and modern computers or mobile devices.

Structure and Compression

SPU files follow a straightforward structure, with a header containing information about the image’s resolution, colors, and other attributes. The actual image data is stored in an uncompressed bitmap format, ensuring efficient access and fast display times. However, SPU files can be compressed using the RLE (Run-Length Encoding) algorithm to reduce file size without compromising image quality. This compression technique is particularly effective for images with large areas of solid colors or repeating patterns. By utilizing RLE, SPU files can significantly reduce their size while maintaining visual fidelity, making them suitable for distribution or storage on limited-capacity media.

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