JPEG is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality.
JPEG codec on {IDF_TARGET_NAME} is an image codec, which is based on the JPEG baseline standard, for compressing and decompressing images to reduce the bandwidth required to transmit images or the space required to store images, making it possible to process large-resolution images. But please note, at one time, the codec engine can only work as either encoder or decoder.
-`Resource Allocation <#resource-allocation>`__ - covers how to allocate JPEG resources with properly set of configurations. It also covers how to recycle the resources when they finished working.
-`Finite State Machine <#finite-state-machine>`__ - covers JPEG workflow. Introduce how jpeg driver uses internal resources and its software process.
-`JPEG Decoder Engine <#jpeg-decoder-engine>`__ - covers behavior of JPEG decoder engine. Introduce how to use decoder engine functions to decode an image (from jpg format to raw format).
-`JPEG Encoder Engine <#jpeg-encoder-engine>`__ - covers behavior of JPEG encoder engine. Introduce how to use encoder engine functions to encode an image (from raw format to jpg format).
-`Performance Overview <#performance-overview>`__ - covers encoder and decoder performance.
-`Pixel Storage Layout for Different Color Formats <#pixel-storage-layout-for-different-color-formats>`__ - covers color space order overview required in this JPEG decoder and encoder.
If the configurations in :cpp:type:`jpeg_decode_engine_cfg_t` is specified, users can call :cpp:func:`jpeg_new_decoder_engine` to allocate and initialize a JPEG decoder engine. This function will return an JPEG decoder handle if it runs correctly. You can take following code as reference.
If a previously installed JPEG engine is no longer needed, it's recommended to recycle the resource by calling :cpp:func:`jpeg_del_decoder_engine`, so that the underlying hardware is released.
If the configurations in :cpp:type:`jpeg_encode_engine_cfg_t` is specified, users can call :cpp:func:`jpeg_new_encoder_engine` to allocate and initialize a JPEG encoder engine. This function will return an JPEG encoder handle if it runs correctly. You can take following code as reference.
If a previously installed JPEG engine is no longer needed, it's recommended to recycle the resource by calling :cpp:func:`jpeg_del_encoder_engine`, so that the underlying hardware is released.
After installing the JPEG decoder driver by :cpp:func:`jpeg_new_decoder_engine`, {IDF_TARGET_NAME} is ready to decode JPEG pictures by :cpp:func:`jpeg_decoder_process`. :cpp:func:`jpeg_decoder_process` is flexible for decoding different types of pictures by a configurable parameter called :cpp:type:`jpeg_decode_cfg_t`.
Moreover, our jpeg decoder api provides a helper function which helps you get the basic information of your given image. Calling :cpp:func:`jpeg_decoder_get_info` would return the picture information structure called :cpp:func:`jpeg_decoder_get_info`. If you already know the picture basic information, this functions is unnecessary to be called.
1. In above code, you should make sure the `bit_stream` and `out_buf` should be aligned by certain rules. We provide a helper function :cpp:func:`jpeg_alloc_decoder_mem` to help you malloc a buffer which is aligned in both size and address.
3. The width and height of output picture would be 16 bytes aligned if original picture is formatted by YUV420 or YUV422. For example, if the input picture is 1080*1920, the output picture will be 1088*1920. That is the restriction of jpeg protocol. Please provide sufficient output buffer memory.
After installing the JPEG encoder driver by :cpp:func:`jpeg_new_encoder_engine`, {IDF_TARGET_NAME} is ready to decode JPEG pictures by :cpp:func:`jpeg_encoder_process`. :cpp:func:`jpeg_encoder_process` is flexible for decoding different types of pictures by a configurable parameter called :cpp:type:`jpeg_encode_cfg_t`.
The format conversions supported by this driver are listed in the table below:
1. In above code, you should make sure the `raw_buf_1080p` and `jpg_buf_1080p` should aligned by calling :cpp:func:`jpeg_alloc_encoder_mem`.
2. The content of `raw_buf_1080p` buffer should not be changed until :cpp:func:`jpeg_encoder_process` returns.
3. The compression ratio depends on the chosen `image_quality` and the content of the image itself. Generally, a higher `image_quality` value obviously results in better image quality but a smaller compression ratio. As for the image content, it is hard to give any specific guidelines, so this question is out of the scope of this document. Generally, the baseline JPEG compression ratio can vary from 40:1 to 10:1. Please take the actual situation into account.
Performance Overview
^^^^^^^^^^^^^^^^^^^^
This section provides some measurements of the decoder and encoder performance. The data presented in the tables below gives the average values of decoding or encoding a randomly chosen picture fragments for 50 times. All tests were performed at a CPU frequency of 360MHz and a SPIRAM clock frequency of 200MHz. Only JPEG related code is run in this test, no other modules are involved (e.g. USB Camera, etc.).
Both decoder and encoder are not cause too much CPU involvement. Only header parse causes CPU source. Calculations related to JPEG compression, such as DCT, quantization, huffman encoding/decoding, etc., are done entirely in hardware.
The encoder and decoder described in this guide use the same uncompressed raw image formats (RGB, YUV). Therefore, the encoder and decoder are not discussed separately in this section. The pixel layout of the following formats applies to the input direction of the encoder and the output direction of the decoder (if supported). The specific pixel layout is shown in the following figure:
RGB888
~~~~~~
In the following picture, each small block means one bit.
The factory function :cpp:func:`jpeg_new_decoder_engine`, :cpp:func:`jpeg_decoder_get_info`, :cpp:func:`jpeg_decoder_process`, and :cpp:func:`jpeg_del_decoder_engine` are guaranteed to be thread safe by the driver, which means, user can call them from different RTOS tasks without protection by extra locks.
When power management is enabled (i.e., :ref:`CONFIG_PM_ENABLE` is set), the system needs to adjust or stop the source clock of JPEG to enter Light-sleep, thus potentially changing the JPEG decoder or encoder process. This might lead to unexpected behavior in hardware calculation. To prevent such issues, entering light sleep is disabled for the time when JPEG encoder or decoder is working.
Whenever the user is decoding or encoding via JPEG (i.e., calling :cpp:func:`jpeg_encoder_process` or :cpp:func:`jpeg_decoder_process`), the driver guarantees that the power management lock is acquired by setting it to :cpp:enumerator:`esp_pm_lock_type_t::ESP_PM_NO_LIGHT_SLEEP`. Once the encoding or decoding is finished, the driver releases the lock and the system can enter light sleep.
