用Matlab实现JPEG编码器代码，含量化因子，标记出压缩前后的文件大小，计算压缩比

以下是一个简单的Matlab代码实现JPEG编码器，并包含了量化因子、文件大小和压缩比的计算：

% 读入原始图像
img = imread('lena.png');

% 色彩空间转换为YCbCr
img_ycbcr = rgb2ycbcr(img);

% 采样
img_ycbcr_subsampled = img_ycbcr(1:2:end, 1:2:end, :);

% 定义量化表和量化因子
quantization_table = [16 11 10 16 24 40 51 61;
                      12 12 14 19 26 58 60 55;
                      14 13 16 24 40 57 69 56;
                      14 17 22 29 51 87 80 62;
                      18 22 37 56 68 109 103 77;
                      24 35 55 64 81 104 113 92;
                      49 64 78 87 103 121 120 101;
                      72 92 95 98 112 100 103 99];

quantization_factor = 10;

% 进行DCT变换和量化
img_dct_quantized = img_ycbcr_subsampled;
for i = 1:8:size(img_ycbcr_subsampled, 1)
    for j = 1:8:size(img_ycbcr_subsampled, 2)
        img_dct = dct2(img_ycbcr_subsampled(i:i+7, j:j+7, :));
        img_dct_quantized(i:i+7, j:j+7, :) = round(img_dct ./ (quantization_table * quantization_factor));
    end
end

% 进行熵编码
img_huffman_encoded = huffmanenco(img_dct_quantized(:), huffmandict([0:255], histcounts(img_dct_quantized(:), 256)));

% 将压缩后的数据解码
img_dct_quantized_decoded = huffmandeco(img_huffman_encoded, huffmandict([0:255], histcounts(img_dct_quantized(:), 256)));

% 进行IDCT变换和反量化
img_ycbcr_subsampled_decoded = img_dct_quantized_decoded;
for i = 1:8:size(img_ycbcr_subsampled, 1)
    for j = 1:8:size(img_ycbcr_subsampled, 2)
        img_idct = idct2(img_dct_quantized_decoded((i-1)*size(img_ycbcr_subsampled,2)+j:i*size(img_ycbcr_subsampled,2)+(j-1), :));
        img_ycbcr_subsampled_decoded(i:i+7, j:j+7, :) = img_idct .* (quantization_table * quantization_factor);
    end
end

% 进行颜色空间转换为RGB
img_reconstructed = ycbcr2rgb(img_ycbcr_subsampled_decoded);

% 计算压缩前后的文件大小和压缩比
original_size = numel(img);
compressed_size = numel(img_huffman_encoded);
compression_ratio = original_size / compressed_size;

% 显示压缩前后的图像
subplot(1,2,1);
imshow(img);
title('Original Image');
subplot(1,2,2);
imshow(img_reconstructed);
title('Reconstructed Image');

% 显示压缩前后的文件大小和压缩比
fprintf('Original Size: %d bytes\n', original_size);
fprintf('Compressed Size: %d bytes\n', compressed_size);
fprintf('Compression Ratio: %f\n', compression_ratio);

请注意，这只是一个简单的实现，可能存在一些性能问题和改进的空间。