Lossless encoding for the non-primary image

[bigcat88]

Discussed in #307

^{Originally posted by Foxify52 November 5, 2024}
The following is an encoder i wrote to take advantage of the multi layer support avif has. Normally when using this code to encode data by itself in the first layer, it can encode and decode perfectly without any issue. as soon as you try and encode data and place it in a layer, it seems to ignore the lossless encoding i specified for those layers. I dont think its an issue with the package as much as a user error but it is one of those edge cases that might now have been accounted for. Any help would be appreciated.

Encoder:

import numpy as np
from PIL import Image
import pillow_heif
import zlib

pillow_heif.register_avif_opener()


def encode_to_image(data):
    binary_data = zlib.compress(data)
    side_length = int(np.ceil(np.sqrt((len(binary_data) + 3) // 4)))
    binary_array = np.pad(
        np.frombuffer(binary_data, dtype=np.uint8),
        (0, side_length**2 * 4 - len(binary_data)),
        mode="constant",
    ).reshape((side_length, side_length, 4))
    return Image.fromarray(binary_array, "RGBA")


base = Image.open("base.jpg")
encoded_images = [encode_to_image(b"Some binary data to encode")]
# Save the image
base.save(
    "output.avif",
    format="AVIF",
    save_all=True,
    primary_index=0,
    append_images=encoded_images,
    quality=-1,  # Required for lossless compression
    chroma=444,  # Required for correct color subsampling
    matrix_coefficients=0,  # Required to convert to RGB color space
)

Decoder:

import zlib

import numpy as np
import pillow_heif
from PIL import Image, ImageSequence

pillow_heif.register_avif_opener()

img = Image.open("output.avif")
bin_data = ImageSequence.Iterator(img)[1].copy()
reshaped_binary_array = np.array(bin_data)
flattened_binary_array = reshaped_binary_array.flatten()
binary_data_with_padding = flattened_binary_array.tobytes()
binary_data_without_padding = zlib.decompress(binary_data_with_padding.rstrip(b"\x00"))
original_binary_data = b"Some binary data to encode"
print(f"Original binary data: {original_binary_data}")
print(f"Decoded binary data: {binary_data_without_padding}")
print(
    f"Does the decoded data match the original data? {binary_data_without_padding == original_binary_data}"
)

[bigcat88]

REMOVED, too tired, wrote a mess

[Foxify52]

I'm not sure if it would be a big inconvenience but could information about this be added into the documentations for future references? I don't believe there's much on configuration for non primary images.

[bigcat88]

I'll do a PR with implementation on the weekend and add it to the documentation.