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Showing posts from August, 2020

Optimizing JPEG decoding

The aim of this blog post is to document my new optimized Arduino/embedded JPEG decoder library. There are already a couple of good Arduino libraries available, so why re-invent the wheel? It's best to take a step back and explain why it's challenging to decode JPEG files on 'small memory' devices, then it will be clearer why I wrote it. Let's say that we want to display a 320x240 image on one of the popular TFT LCD displays. A 320x240x16-bit (RGB565) image requires 153K bytes of storage. For a device like the Arduino Mega2560, it would be impossible to fit the whole image in its 8K of internal RAM. Luckily the way JPEG baseline images are stored, it's not necessary to hold the entire image in memory.  The image is divided into MCUs (minimum coded units) which all derive from a basic 8x8 Discrete Cosine Transform. Each of these blocks can be decoded in sequence and sent to a SPI LCD display individually. SPI LCDs have their own internal frame buffer memory. The…