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85 lines
2.8 KiB
ReStructuredText
85 lines
2.8 KiB
ReStructuredText
RLE Decoder
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===========
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The RLE Decoder is a part of LVGL that is responsible for decoding RLE
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compressed original LVGL binary files. The file name suffix is always set
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to '.rle' and the file header is added with another 64-bit value that
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includes a 32-bit magic number and 32-bit RLE information.
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The decoder supports both variable and file as image sources. The original
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binary data is directly decompressed to RAM, and further decoded by LVGL's
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built-in decoder.
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Benefits
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--------
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Based on test result from a watch project. Most of the images can be compressed
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to save more than 70% space as show in below statistic. It shows the file count
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of every compress level. For rare conditions, RLE compress may increase the file
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size if there's no large repetition in data.
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.. image:: /libs/rle-compress-statistics.png
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:alt: RLE compress statistics from a watch project
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:align: center
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Theory
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------
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The RLE algorithm is a simple compression algorithm that is based on the fact that
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the for many pixels, the color is the same. The algorithm simply counts how many
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repeated data are there and store the count value and the color value.
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If the coming pixels are not repeated, it stores the non-repeat count value and
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original color value. For more details, the script used to compress the image
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can be found from `lvgl/script/LVGLImage.py`.
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.. code:: python
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def rle_compress(self, data: bytearray, blksize: int, threshold=16):
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index = 0
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data_len = len(data)
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compressed_data = []
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while index < data_len:
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memview = memoryview(data)
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repeat_cnt = self.get_repeat_count(
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memview[index:], blksize)
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if repeat_cnt == 0:
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# done
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break
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elif repeat_cnt < threshold:
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nonrepeat_cnt = self.get_nonrepeat_count(
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memview[index:], blksize, threshold)
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ctrl_byte = uint8_t(nonrepeat_cnt | 0x80)
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compressed_data.append(ctrl_byte)
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compressed_data.append(
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memview[index: index + nonrepeat_cnt*blksize])
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index += nonrepeat_cnt * blksize
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else:
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ctrl_byte = uint8_t(repeat_cnt)
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compressed_data.append(ctrl_byte)
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compressed_data.append(memview[index: index + blksize])
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index += repeat_cnt * blksize
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return b"".join(compressed_data)
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Usage
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-----
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To use the RLE Decoder, ensure that `LV_USE_RLE` is defined and set to `1`.
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The RLE image can be used same as other images.
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.. code:: c
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lv_image_set_src(img, "path/to/image.rle");
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Generate RLE images
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-------------------
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The image can be directly generated using script `lvgl/script/LVGLImage.py`
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.. code:: bash
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./script/LVGLImage.py --ofmt RLE --cf I8 cogwheel.png
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