JPEG Decode SW

[中文]

1. JPEG Soft Decoding Process

The main process of JPEG soft decoding is as follows:

  1. Pre decoding, parsing the header of a JPEG data to obtain image format, image length and width, quantization table, and Huffman table;

  2. Search for quantization tables and Huffman tables to parse a block of data;

  3. Convert a block data into the image format that needs to be output;

  4. Write the converted image data into the output image;

  5. Repeat 2-4 until an exception occurs or decoding is complete;

jpeg_decode sw

Figure 1. jpeg soft decode process

2. Memory requirements

JPEG soft decoding relies on threads within 1K, and the main buffers and purposes required are as follows:

  1. A 10240 buffer for storing Huffman tables and intermediate data processing;

  2. A buffer of size 0xB0, used to store intermediate running pointers and variables;

  3. A 16 * 16 * 2 sized buffer for image rotation after soft decoding;

3. Use of JPEG soft decoding module

  1. Call bk_jpeg_dec_sw_init to initialize the soft decoding module;

  2. Call bk_jpeg_dec_sw_start to decode the incoming image;

  3. Call bk_jpeg_dec_sw_deinit to release the internal buffer of soft decoding;

Please refer to API Reference : for function parameters.

4. One time decoding function

For some scenarios, a one-time decoding interface bk_jpeg_dec_sw_start_one_time is provided, Before using this interface, there is no need to call bk_jpeg_dec_sw_init, and the buffer is passed in externally;

If NULL is passed in externally, it will be automatically requested by the interface internally; Automatically release the internal requested buffer after decoding is completed;

Note

This interface can be decoded in parallel with the soft decoding module, but the decoding speed will be reduced when parallel;

5. Decoding speed optimization

When DTCM is sufficient, CONFIG_SOFTWARE_DECODE_SRAM_MAPPING can be set to y on CP1 and CP2 to speed up decoding; The optimization method is to put the decoding thread and part of the buffer used for decoding on dtcm, and at the same time, change the original decoding block and save it on psram to buffer 16 lines and then save them together on psram, reducing the time of frequent psram address switching in actual operation;

Note

The soft decoding and rotation modules cannot work at the same time because the buffer used to buffer 16 lines in soft decoding is the buffer used in the rotation module;

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