PNG Lossless Compression Encoder

Overview

The PNG-E core implements a lossless image compression engine compliant with the Portable Network Graphics (PNG) file format specified in the ISO/IEC 15948 and RFC 2083 standards.

The encoder core can compress greyscale or color (RGB) images, with 8 and 16 bits per color, with or without alpha transparency, and produces complete PNG files. It exhibits excellent compression efficiency thanks to its ability to automatically and dynamically choose the optimal prediction filter per line. It supports all four prediction filters, LZ77 with a configurable History Window, and Static Huffman tables, and computes CRC and Adler32 checksums as provisioned by the standard. The core does not currently support the interlaced mode, Dynamic Huffman tables, and images with less than 8 bits per color, but these can be added on request.

The easy-to-use PNG-E core interfaces to the system via standardized AMBA® interfaces: it accepts images and outputs compressed data via AXI4-Stream interfaces and provides access to its control and status registers via a 32-bit APB interface. After its registers are programmed with the image dimensions and color format, the core can encode an arbitrary number of images without requiring any assistance or action from the system. The core provides one interrupt signal per direction to facilitate integration with a DMA engine. These interrupts are asserted when the input or output FIFO occupancy reaches a run-time programmable threshold.

Key Features

Efficient PNG Encoder

Supports 8- and 16-bit greyscale, 24- and 48-bit truecolor, with or without alpha transparency
Outputs complete PNG files compliant with the ISO/IEC 15948 and RFC 2083 standards
Optimizes compression ratio by automatically selecting the best prediction filter for each line
Implements all four prediction filters, LZ77 with a configurable history window, and Huffman encoding with static tables, and calculates Adler32 and CRC checksums

Easy to Use and Integrate

Run-time programmable image format parameters
Automatic program-once encode-many operation
AXI4-Stream Interfaces for image and compressed data
AXI-Lite or APB for control and Status register access
Interrupts/DMA handshaking signals trigger based on programmable FIFO thresholds

Throughput & Latency

One byte per cycle throughput

Higher throughput versions can be made available on request

Ultra-low latency: one image line plus 40 clock cycles

Synthesis-Time Configuration Options

Maximum image resolution and bytes per pixel
LZ77 history window size
Input & output FIFO depths

Block Diagram

Applications

A wide range of SoC designs using PNG files can benefit from this custom-hardware decompression engine's high performance and lower power. End applications range from wearables to data center acceleration and aerospace imaging.

Deliverables

Consistent with CAST’s quality standards, this core has been rigorously verified, is LINT-clean and scan-ready, and is delivered with everything required for a trouble-free implementation. It is available in Verilog RTL source code or as a targeted FPGA netlist, and its deliverables include a sophisticated self-checking testbench, a bit-accurate software model, sample synthesis and simulation scripts, IP-XACT register descriptions and comprehensive documentation.

Technical Specifications

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