JPG decoder

JPG decoder

Allow the use of JPG images in LVGL. Besides that it also allows the use of a custom format, called Split JPG (SJPG), which can be decided in more optimal way on embedded systems.

Overview

• Supports both normal JPG and the custom SJPG formats.
• Decoding normal JPG consumes RAM with the size fo the whole uncompressed image (recommended only for devices with more RAM)
• SJPG is a custom format based on “normal” JPG and specially made for LVGL.
• SJPG is ‘split-jpeg’ which is a bundle of small jpeg fragments with an sjpg header.
• SJPG size will be almost comparable to the jpg file or might be a slightly larger.
• File read from file and c-array are implemented.
• SJPEG frame fragment cache enables fast fetching of lines if availble in cache.
• By default the sjpg image cache will be image width * 2 * 16 bytes (can be modified)
• Currently only 16 bit image format is supported (TODO)
• Only the required partion of the JPG and SJPG images are decoded, therefore they can’t be zoomed or rotated.

Usage

If enabled in lv_conf.h by LV_USE_SJPG LVGL will register a new image decoder automatically so JPG and SJPG files can be directly used as image sources. For example:

lv_img_set_src(my_img, "S:path/to/picture.jpg");

Note that, a file system driver needs to registered to open images from files. Read more about it here or just enable one in lv_conf.h with LV_USE_FS_...

Converter

Converting JPG to C array

• Use lvgl online tool https://lvgl.io/tools/imageconverter
• Color format = RAW, output format = C Array

Converting JPG to SJPG

python3 and the PIL library required. (PIL can be installed with pip3 install pillow)

To create SJPG from JPG:

• Copy the image to convert into lvgl/scripts
• cd lvgl/scripts
• python3 jpg_to_sjpg.py image_to_convert.jpg. It creates both a C files and an SJPG image.

The expected result is:

Conversion started...

Input:
        image_to_convert.jpg
        RES = 640 x 480

Output:
        Time taken = 1.66 sec
        bin size = 77.1 KB
        walpaper.sjpg           (bin file)
        walpaper.c              (c array)

All good!

Example

API

警告

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