JPEG Encoder bug - matlab

Question

I am trying to make my own JPEG encoder (relying on only low level operations). I am mostly done, but I get an error where the image is coming out all wrong. I think it is due to my understanding of the format itself. I have a 16x16 test image, and I use the header straight from the image (I replaced the quantization and huffman tables to match the ones I am using, but I think I did that correctly since I could pretty clearly make out what I need to replace and what I leave). My problem is understanding the data part after SOS. So, we have our 8x8, DCT-II'd, Quantizied block, we read it zig-zag and grab the first digit and its our DC. We then see if its positive or negative and its general length. (length = ceil(log2(abs(x))). We then use the length to pull the huffman code from the table, and then write those bits (I have a buffer so every time I get 8 bits I write a byte). Then you go through the RLE of AC (which we got from traversing the rest of the 8x8 in zig zag and writing how many 0's we saw before). If the number of zeros before is > 16 I put the 0xf0, do I need to then put a val of 0? Or do I just leave it? assuming its a normal number (take 25 for example), I write the huffman code from the run//size key dict, then I just put 011001 (2's comp 25) right? Then I move on till I hit 0's till the end and put the 0000/0000 code in (which let's say it translates to 00 so we just append 00 to the file). Then I rinse and repeat? Do I need to do anything between 8x8 blocks? Does the order go DC - Y comp, AC - Y comp, DC - Cb comp, AC Cb comp, DC Cr comp, AC - Cr comp, next block? Do I do anything between blocks? And for the DC "previous diff" situation, does that mean previous DC val in that component? Like the previous Y comp's DC? At the end I pad 1's till I complete a byte and then write the EOI tag right?
[for loop handling the 8x8, quant, dft ect.]
% Huffman Encoding
% DC Encoding
if channel == 1
val = freq(1) - prev_DC_Y;
elseif channel == 2
val = freq(1) - prev_DC_Cb;
else
val = freq(1) - prev_DC_Cr;
end
if val ~= 0
size_of_bits = log2(abs(val));
size_of_bits = ceil(size_of_bits);
else
size_of_bits = 0;
end
if channel == 1
num_bits_to_append = cell2mat(dc_lum{size_of_bits+1}(1));
bits_to_append = cell2mat(dc_lum{size_of_bits+1}(2));
else
num_bits_to_append = cell2mat(dc_cbcr{size_of_bits+1}(1));
bits_to_append = cell2mat(dc_cbcr{size_of_bits+1}(2));
end
for i1 = 1:num_bits_to_append
bits(bits_on_deck+1) = bits_to_append(i1);
bits_on_deck = bits_on_deck + 1;
end
if val < 0
bits(bits_on_deck+1) = 1;
bits_on_deck = bits_on_deck + 1;
val = abs(val) - 1;
else
bits(bits_on_deck+1) = 0;
bits_on_deck = bits_on_deck + 1;
end
for i1 = 1:size_of_bits
if bitand(bitshift(1,i1-1), val) ~= 0
bits(bits_on_deck+1) = 1;
bits_on_deck = bits_on_deck + 1;
else
bits(bits_on_deck+1) = 0;
bits_on_deck = bits_on_deck + 1;
end
while bits_on_deck >= 8
manage_bits(fileID, bits);
bits_on_deck = bits_on_deck - 8;
end
end
if channel == 1
prev_DC_Y = val;
elseif channel == 2
prev_DC_Cb= val;
else
prev_DC_Cr = val;
end
% AC Encoding
i1 = 1;
code = 1;
while code ~= 0
code = order(i1)
val = order(i1+1)
size_of_bits = ceil(log2(abs(val)));
if channel == 1
num_bits_to_append = cell2mat(ac_lum{code+1}(1));
bits_to_append = cell2mat(ac_lum{code+1}(2));
else
num_bits_to_append = cell2mat(ac_cbcr{code+1}(1))
bits_to_append = cell2mat(ac_cbcr{code+1}(2))
end
for i2 = 1:num_bits_to_append
bits(bits_on_deck+1) = bits_to_append(i2);
bits_on_deck = bits_on_deck + 1;
end
while bits_on_deck >= 8
manage_bits(fileID, bits);
bits_on_deck = bits_on_deck - 8;
end
if code == 0 break
end
if val < 0
bits(bits_on_deck+1) = 1;
bits_on_deck = bits_on_deck + 1;
val = abs(val) - 1;
else
bits(bits_on_deck+1) = 0;
bits_on_deck = bits_on_deck + 1;
end
for i2 = 1:size_of_bits
if bitand(bitshift(1,i2-1), val) ~= 0
bits(bits_on_deck+1) = 1;
bits_on_deck = bits_on_deck + 1;
else
bits(bits_on_deck+1) = 0;
bits_on_deck = bits_on_deck + 1;
end
while bits_on_deck >= 8
manage_bits(fileID, bits);
bits_on_deck = bits_on_deck - 8;
end
end
i1 = i1 + 2;
end
end
end
end
if bits_on_deck ~= 0
while bits_on_deck < 8
bits(bits_on_deck+1) = 1;
bits_on_deck = bits_on_deck + 1;
end
end
manage_bits(fileID, bits);
fwrite(fileID, [0xff, 0xd9]);
fclose(fileID);
Target
Actual