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 decoded 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 available in cache.
By default the sjpg image cache will be image width * 2 * 16 bytes (can be modified)
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¶

Load an SJPG image¶

#include "../../lv_examples.h"
#if LV_USE_SJPG && LV_BUILD_EXAMPLES

/**
 * Load an SJPG image
 */
void lv_example_sjpg_1(void)
{
    lv_obj_t * wp;

    wp = lv_img_create(lv_scr_act());
    /* Assuming a File system is attached to letter 'A'
     * E.g. set LV_USE_FS_STDIO 'A' in lv_conf.h */
    lv_img_set_src(wp, "A:lvgl/examples/libs/sjpg/small_image.sjpg");
}

#endif

#!/opt/bin/lv_micropython -i
import lvgl as lv
import display_driver
import fs_driver

fs_drv = lv.fs_drv_t()
fs_driver.fs_register(fs_drv, 'S')

wp = lv.img(lv.scr_act())
# The File system is attached to letter 'S'

wp.set_src("S:small_image.sjpg")
wp.center()

API¶

Functions

void lv_split_jpeg_init(void)¶