.. _jpeg_decode:

JPEG decode
======================

Overview
--------

JPEG decoding example project demonstrates how to decode a JPG file using either JPEG module or software jpeg library

Board Setting
-------------

- Connect the **USB disk**  to the development board ``usb0`` (Attention: The `Cluster Size` of the USB disk can't exceed 32KB, it is recommended that the format parameter is FAT32/32KB), or insert the **SD memory card**  into the development board ``SD card``

  .. image:: ../../../../../samples/image/common/doc/UDisk_Format.png
     :alt: UDisk_Format

- Connect **LCD screen**  to development board

Configuration
-------------

- File `CMakeLists.txt`.

  - ``set (JPEG_USE_SDCARD 1)``  to read jpeg file (.jpg) from SD card or ``set (JPEG_USE_UDISK 1)``  to read jpeg file (.jpg) from u-disk. These options are exclusive.

  - ``set (JPEG_HW_MODE 1)``  to decode jpeg file with JPEG module; ``set (JPEG_HW_MODE 0)``  to decode with software library

Supported picture formats
-------------------------

- Supports standard JPEG images：

  JPEG picture format components：SOI（File header）+APP0（Image identification information）+ DQT（Define quantification table）+ SOF0（Basic image information）+ DHT（Define Huffman table） + DRI（Define restart interval）+ SOS（Scan line start）+ EOI（End of document），the SOI is composed of two hexadecimal bytes (ffd8)。
- The sampling factor supported by the JPEG picture is：YUV420、YUV422H、YUV422V、YUV444、YUV400，Such as a JPG image in 800 * 400 YUV420 format，Through the worm EXIF viewer`Graphic insect EXIF viewer official website <https://exif.tuchong.com//>`_available：

    FileType	        JPEG
    FileTypeExtension	jpg
    MIMEType	        image/jpeg
    ImageWidth	        800
    ImageHeight	        480
    EncodingProcess	    Baseline DCT, Huffman coding
    BitsPerSample	    8
    ColorComponents	    3
    YCbCrSubSampling  **YCbCr4:2:0 (2 2)**

Image size and resolution supported by this routine
---------------------------------------------------

- jpeg file size is expected to be less than 500KB, image resolution is expected to be less than 1024 * 768, defined in common/inc/file_op.h

Running the example
-------------------

- Configure storage in CMakeLists.txt

- Put jpeg files (with .jpg) into target storage

- Connect LCD pannel and storage to board

- Power on the board

- Download built elf into development board and run the demo

- If everything goes well, jpeg files stored in the target storage will be show on LCD pannel one by one.