PKM is a simple container format for ETC-compressed textures, widely used on Android and OpenGL ES platforms. This article provides a complete guide to understanding PKM format structure, ETC/ETC2 compression versions, and how to use them effectively.

What is PKM Format?

PKM (short for 'PoKeMon', named by Ericsson) is a simple container format designed specifically for storing ETC-compressed texture data. Unlike complex formats like KTX or DDS, PKM has a minimal structure with just a 16-byte header followed by compressed image data.

The format was created to provide a lightweight way to store and load ETC-compressed textures on mobile devices. Its simplicity makes it easy to implement and parse, with virtually no overhead beyond the compressed texture data itself.

PKM File Structure

16-byte header: Magic number (PKM), version, format, dimensions
Compressed texture data: Raw ETC/ETC2 blocks

ETC Compression Versions

There are two major versions of ETC compression, each with different capabilities:

ETC1 (Ericsson Texture Compression 1)

The original ETC standard, supporting only RGB color channels with 4:1 fixed compression ratio. Widely supported on Android devices since OpenGL ES 2.0.

Support: All OpenGL ES 2.0+ devices

Compression Ratio: 4:1 fixed (4 bits per pixel)

ETC2 (Ericsson Texture Compression 2)

Improved successor to ETC1, adding RGBA support with multiple format options. Provides better quality and alpha channel support while maintaining backward compatibility.

Support: OpenGL ES 3.0+, Vulkan

Compression Ratio: 4:1 to 8:1 depending on format

PKM Format Variants

PKM can contain different ETC format types:

ETC1 RGB

Original format, RGB only, no alpha channel

ETC2 RGB

Improved RGB compression with better quality than ETC1

ETC2 RGBA

Full RGBA support with 8:1 compression (8 bits per pixel)

ETC2 RGB + Punch-through Alpha

RGB with 1-bit alpha, maintaining 4:1 compression

Advantages of PKM/ETC

Universal Android Support

ETC1 is guaranteed to be supported on all Android devices, making it the most portable compression format for Android development.

Fixed Compression Ratio

Predictable memory usage with consistent 4:1 or 8:1 compression ratios, simplifying memory budget calculations.

Good Quality-to-Size Ratio

ETC2 provides excellent visual quality for most texture types, comparable to desktop formats like DXT/BCn.

Hardware Decoding

Direct GPU decompression with zero CPU overhead during rendering, enabling efficient texture sampling.

Limitations and Considerations

ETC1 Quality Limits

ETC1 can show visible artifacts on textures with high-frequency detail or smooth gradients. Consider using ETC2 for better quality.

No Alpha in ETC1

ETC1 doesn't support alpha channels. You need to use ETC2 RGBA or store alpha separately if targeting OpenGL ES 2.0 devices.

Limited Advanced Features

PKM doesn't support mipmaps, cubemaps, or texture arrays. Use KTX or KTX2 formats for these advanced features.

How to Use PKM Format

Using PKM files in your projects is straightforward:

Encoding to PKM

Upload your PNG/JPG image (must have power-of-2 or multiple-of-4 dimensions)
Choose between ETC1 RGB or ETC2 RGB/RGBA based on your needs
Download the generated .pkm file and integrate it into your project

Try PKM Encoder

Decoding PKM Files

Upload your .pkm file to verify its contents
Preview the decompressed texture and check quality
Export to PNG if you need to edit or inspect the texture

Try PKM Decoder

Best Practices

Use ETC1 RGB for maximum compatibility across all Android devices (OpenGL ES 2.0+)
Prefer ETC2 RGBA when targeting modern devices (OpenGL ES 3.0+) that need alpha channels
Ensure texture dimensions are multiples of 4 (ETC1) or power-of-2 for best quality
For textures requiring alpha on older devices, consider using separate grayscale textures
Test compressed textures on actual target devices - quality perception varies by content type

Conclusion

PKM with ETC/ETC2 compression is the standard choice for Android and mobile OpenGL ES development. Its universal support, predictable performance, and good quality make it an excellent default for mobile texture compression.

Our online PKM encoder and decoder tools make it easy to work with ETC-compressed textures without installing command-line tools or SDKs. Whether you're creating new textures or inspecting existing ones, these tools help you optimize your mobile graphics pipeline.

PKM and ETC Format: Complete Guide