JPG decoder for LVGL

Overview

• lv_lib_split_jpg supports both normal jpg and the custom sjpg formats.
• Decoding normal jpg consumes RAM with the site 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.
• lv_sjpg can open and handle multiple files at same time.
• File read from disk (fread) and c-array is implemented.
• SJPEG frame fragment cache enables fast fetching of lines if availble in cache.
• By default the sjpg image cache will be xres * 2 * 16 bytes (can be modified)
• Currently only 16 bit image format is supported (to do)

Getting started

• Clone this repository next to lvgl with git clone https://github.com/lvgl/lv_lib_split_jpg.git
• If you need to open (S)JPG files use lv_fs_if or add your own file system drivers.
  • With lv_fs_if add these line to the end of lv_conf.h to enable "PC file system" handling

#define LV_USE_FS_IF	1
#if LV_USE_FS_IF
#  define LV_FS_IF_FATFS    '\0'
#  define LV_FS_IF_PC     'S'
#endif  

• For fast simulatneous multi jpg/sjpg rendering in expense of additional RAM, you can modify LV_IMG_CACHE_DEF_SIZE to 2 or above if testing in simulator or using it in devices like raspberry pi etc.
• To open large JPG image a lot of dymaic memory is required. Be sure to set LV_MEM_SIZE to large enough value. In simulator (4 * 1024 * 1024) shiuld be a good start.

Example code

#include <lvgl/lvgl.h>
#include "lv_lib_split_jpg/lv_sjpg.h"

LV_IMG_DECLARE( small_image_sjpg );
LV_IMG_DECLARE( wallpaper_jpg );

void demo_jpg_sjpg( void )
{
  lv_fs_if_init();
  lv_obj_t * img1;
  lv_obj_t * img2;

  lv_split_jpeg_init();
  img1 = lv_img_create(lv_scr_act(), NULL);
  img2 = lv_img_create(lv_scr_act(), NULL);

  //jpg from c array
  lv_img_set_src( img1,  &wallpaper_jpg);

  //sjpg from file (with lv_fs)
  //On Windows
  lv_img_set_src(img2,  "S.\\lv_lib_split_jpg\\example\\images\\small_image.sjpg"); 
  
  //On Linux
  //lv_img_set_src(img2,  "S/lv_lib_split_jpg/example/images/small_image.sjpg"); 
}

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 PIL library required

To create SJPG from JPG:

1. Drag and drop a jpeg image on top of the jpg_to_sjpg.py
2. Run the python script on shell with jpeg filename as argument. It should generate filename.c and filename.sjpg files.

python3 jpg_to_sjpg.py wallpaper.jpg

Expected result:

Conversion started...

Input:
        walpaper.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!