Debugging the PNG Reader — Handmade Hero — Episode Guide

0:01Recap and set the stage for the day debugging our PNG reader

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0:01Recap and set the stage for the day debugging our PNG reader

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0:01Recap and set the stage for the day debugging our PNG reader

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1:01On starting by making our decompressor produce the correct number of bytes from our input stream

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1:01On starting by making our decompressor produce the correct number of bytes from our input stream

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1:01On starting by making our decompressor produce the correct number of bytes from our input stream

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4:14Approaching problems either by working from a spec, or comparing our implementation with a working version

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4:14Approaching problems either by working from a spec, or comparing our implementation with a working version

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4:14Approaching problems either by working from a spec, or comparing our implementation with a working version

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6:00Run the program on gimp_test.png until a crash caused by the Dest pointer's attempt to write into memory

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6:00Run the program on gimp_test.png until a crash caused by the Dest pointer's attempt to write into memory

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6:00Run the program on gimp_test.png until a crash caused by the Dest pointer's attempt to write into memory

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13:25Let ParsePNG() proceed to the next iteration of the loop, to see another length code 258

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13:25Let ParsePNG() proceed to the next iteration of the loop, to see another length code 258

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13:25Let ParsePNG() proceed to the next iteration of the loop, to see another length code 258

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15:38Compare our PNGLengthExtra and PNGDistExtra tables with the DEFLATE spec¹ to ensure they look right

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15:38Compare our PNGLengthExtra and PNGDistExtra tables with the DEFLATE spec¹ to ensure they look right

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15:38Compare our PNGLengthExtra and PNGDistExtra tables with the DEFLATE spec¹ to ensure they look right

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21:103.2.5 Compressed blocks (length and distance codes)²

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21:103.2.5 Compressed blocks (length and distance codes)²

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21:103.2.5 Compressed blocks (length and distance codes)²

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23:10Scrutinise the length table building code in ParsePNG()

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23:10Scrutinise the length table building code in ParsePNG()

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23:10Scrutinise the length table building code in ParsePNG()

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25:14Assert in ParsePNG() that the LenCount <= number of items in the LitLenDistTable

25:33Continue to scrutinise the Huffman decoding code in ParsePNG()

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25:33Continue to scrutinise the Huffman decoding code in ParsePNG()

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25:33Continue to scrutinise the Huffman decoding code in ParsePNG()

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28:24Assert in ParsePNG() that the DecompressedPixelsEnd does not get exceeded

29:21Run it and hit our Source + Len assertion

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29:21Run it and hit our Source + Len assertion

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29:21Run it and hit our Source + Len assertion

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30:13Consider investigating the Huffman table calculation, before determining to nail down the literal length stuff

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30:13Consider investigating the Huffman table calculation, before determining to nail down the literal length stuff

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30:13Consider investigating the Huffman table calculation, before determining to nail down the literal length stuff

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31:24Produce a smaller image that exhibits the bug

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31:24Produce a smaller image that exhibits the bug

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31:24Produce a smaller image that exhibits the bug

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39:27Note that our bug exhibiting image contains multiple IDAT chunks

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39:27Note that our bug exhibiting image contains multiple IDAT chunks

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39:27Note that our bug exhibiting image contains multiple IDAT chunks

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42:00Produce a 64×64 pixel image that probably can't be PNG compressed

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42:00Produce a 64×64 pixel image that probably can't be PNG compressed

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42:00Produce a 64×64 pixel image that probably can't be PNG compressed

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44:52Run it on gimp_64x64.png to see that the image contains two IDAT chunks and exhibits the crash

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44:52Run it on gimp_64x64.png to see that the image contains two IDAT chunks and exhibits the crash

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44:52Run it on gimp_64x64.png to see that the image contains two IDAT chunks and exhibits the crash

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45:205.3 "Chunk layout" and 11.2.4 "IDAT Image data"³

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45:205.3 "Chunk layout" and 11.2.4 "IDAT Image data"³

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45:205.3 "Chunk layout" and 11.2.4 "IDAT Image data"³

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48:5510.2 Compression of the sequence of filtered scanlines⁴

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48:5510.2 Compression of the sequence of filtered scanlines⁴

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48:5510.2 Compression of the sequence of filtered scanlines⁴

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50:17Run it and hit an assertion in HuffmanDecode() due to an invalid symbol

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50:17Run it and hit an assertion in HuffmanDecode() due to an invalid symbol

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50:17Run it and hit an assertion in HuffmanDecode() due to an invalid symbol

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53:28Step through ConsumeSize() to realise that PeekBits() doesn't correctly operate across multiple IDAT chunks

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53:28Step through ConsumeSize() to realise that PeekBits() doesn't correctly operate across multiple IDAT chunks

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53:28Step through ConsumeSize() to realise that PeekBits() doesn't correctly operate across multiple IDAT chunks

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54:09Fix PeekBits() and ConsumeSize() to correctly operate across multiple IDAT chunks

57:30Run it and again crash in HuffmanDecode() on an invalid symbol, although we did process more bits

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57:30Run it and again crash in HuffmanDecode() on an invalid symbol, although we did process more bits

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57:30Run it and again crash in HuffmanDecode() on an invalid symbol, although we did process more bits

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59:05Crash ScriptedSandbox64.exe

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59:05Crash ScriptedSandbox64.exe

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59:05Crash ScriptedSandbox64.exe

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59:52Continue to investigate our bug exhibited by gimp_64x64.png

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59:52Continue to investigate our bug exhibited by gimp_64x64.png

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59:52Continue to investigate our bug exhibited by gimp_64x64.png

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1:02:51Assert at the end of ParsePNG() that the Dest == DecompressedPixels

1:03:16Run it and do not hit that assertion

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1:03:16Run it and do not hit that assertion

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1:03:16Run it and do not hit that assertion

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1:03:34Determine that gimp_64x64.png crashes 9 bytes from the end, and investigate why

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1:03:34Determine that gimp_64x64.png crashes 9 bytes from the end, and investigate why

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1:03:34Determine that gimp_64x64.png crashes 9 bytes from the end, and investigate why

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1:05:30Make PeekBits() print out BitBufferBeforeAdvance if it needs to advance the buffer

1:07:05Run it to see that there are 12 bits in the buffer but, more pressingly, that we do not handle fixed Huffman compressed chunks

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1:07:05Run it to see that there are 12 bits in the buffer but, more pressingly, that we do not handle fixed Huffman compressed chunks

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1:07:05Run it to see that there are 12 bits in the buffer but, more pressingly, that we do not handle fixed Huffman compressed chunks

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1:08:06Enable ParsePNG() to handle fixed Huffman compressed chunks⁵

1:17:51Step into ParsePNG() and inspect the LitLenDistTable comparing it with the PNG spec⁶

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1:17:51Step into ParsePNG() and inspect the LitLenDistTable comparing it with the PNG spec⁶

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1:17:51Step into ParsePNG() and inspect the LitLenDistTable comparing it with the PNG spec⁶

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1:19:17Determine to perform some difference tests

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1:19:17Determine to perform some difference tests

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1:19:17Determine to perform some difference tests

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1:21:03Grab a screenshot of the stream⁷

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1:21:03Grab a screenshot of the stream⁷

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1:21:03Grab a screenshot of the stream⁷

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1:22:43Run okay on our captured screenshot

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1:22:43Run okay on our captured screenshot

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1:22:43Run okay on our captured screenshot

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1:23:06Q&A

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1:23:06Q&A

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1:23:06Q&A

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1:23:38mtsmox Q: Yeah, same bug

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1:23:38mtsmox Q: Yeah, same bug

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1:23:38mtsmox Q: Yeah, same bug

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1:23:52vaualbus Q: You could use the ray casting code for save image files so we can see what happens. It should be a quick thing to do!

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1:23:52vaualbus Q: You could use the ray casting code for save image files so we can see what happens. It should be a quick thing to do!

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1:23:52vaualbus Q: You could use the ray casting code for save image files so we can see what happens. It should be a quick thing to do!

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1:24:12Run the PNG reader successfully on our ray tracing image

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1:24:12Run the PNG reader successfully on our ray tracing image

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1:24:12Run the PNG reader successfully on our ray tracing image

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1:25:49Credit rooctag for clarifying our understanding of the Paeth filter

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1:25:49Credit rooctag for clarifying our understanding of the Paeth filter

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1:25:49Credit rooctag for clarifying our understanding of the Paeth filter

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1:26:33xxthebigfoxx Q: Did you purposely screenshot my message saying you are handsome?

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1:26:33xxthebigfoxx Q: Did you purposely screenshot my message saying you are handsome?

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1:26:33xxthebigfoxx Q: Did you purposely screenshot my message saying you are handsome?

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1:26:42frostyninja Q: I think he meant save out the decoded PNGs out as a raw BMP so we can see the result?

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1:26:42frostyninja Q: I think he meant save out the decoded PNGs out as a raw BMP so we can see the result?

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1:26:42frostyninja Q: I think he meant save out the decoded PNGs out as a raw BMP so we can see the result?

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1:27:24Pull WriteImage() in from ray.cpp and make ParsePNG() return an image_u32 for us to write out

1:32:27Inspect our written image to see that it is upside-down and incorrectly coloured

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1:32:27Inspect our written image to see that it is upside-down and incorrectly coloured

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1:32:27Inspect our written image to see that it is upside-down and incorrectly coloured

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1:33:15Rename WriteImage() to WriteImageTopDownRGBA() and enable it to swap the rows, introducing SwapRAndB() to swap those colour channels

1:39:47Compare our written image with the original, to see that it is off-by-1 pixel vertically

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1:39:47Compare our written image with the original, to see that it is off-by-1 pixel vertically

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1:39:47Compare our written image with the original, to see that it is off-by-1 pixel vertically

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1:43:05Fix typo in WriteImageTopDownRGBA()

1:43:20Compare our written image with the original to see that they match

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1:43:20Compare our written image with the original to see that they match

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1:43:20Compare our written image with the original to see that they match

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1:44:09bobby1up Q: Isn't it true that the compiler's optimizations of your code and the way the CPU works can sometimes cause instructions to happen in a different order than you wrote them? If so, won't that screw up profiling measurements?

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1:44:09bobby1up Q: Isn't it true that the compiler's optimizations of your code and the way the CPU works can sometimes cause instructions to happen in a different order than you wrote them? If so, won't that screw up profiling measurements?

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1:44:09bobby1up Q: Isn't it true that the compiler's optimizations of your code and the way the CPU works can sometimes cause instructions to happen in a different order than you wrote them? If so, won't that screw up profiling measurements?

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1:44:43tavqua Q: Can you quantify how fast you type?

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1:44:43tavqua Q: Can you quantify how fast you type?

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1:44:43tavqua Q: Can you quantify how fast you type?

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1:44:59jacksonbanan Q: Would you say that 4coder is the ideal text editor?

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1:44:59jacksonbanan Q: Would you say that 4coder is the ideal text editor?

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1:44:59jacksonbanan Q: Would you say that 4coder is the ideal text editor?

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1:45:32jacksonbanan Q: Also, how often do you work out? You're buff

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1:45:32jacksonbanan Q: Also, how often do you work out? You're buff

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1:45:32jacksonbanan Q: Also, how often do you work out? You're buff

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1:45:40lkalinovcic Q: In the BMP write routine, I think you have a bug. You advance Row0 too soon

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1:45:40lkalinovcic Q: In the BMP write routine, I think you have a bug. You advance Row0 too soon

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1:45:40lkalinovcic Q: In the BMP write routine, I think you have a bug. You advance Row0 too soon

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1:45:44enemymouse Q: Double-check the shifting is not a GIMP paster error

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1:45:44enemymouse Q: Double-check the shifting is not a GIMP paster error

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1:45:44enemymouse Q: Double-check the shifting is not a GIMP paster error

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1:45:57jnog92 Q: Just joined a week ago and haven't had time to go through all the series. Can you say in overall what have you done on the project and what is left to do?

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1:45:57jnog92 Q: Just joined a week ago and haven't had time to go through all the series. Can you say in overall what have you done on the project and what is left to do?

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1:45:57jnog92 Q: Just joined a week ago and haven't had time to go through all the series. Can you say in overall what have you done on the project and what is left to do?

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1:47:10bbbyan Q: Do you have any project recommendations for beginner-intermediate C programmers?

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1:47:10bbbyan Q: Do you have any project recommendations for beginner-intermediate C programmers?

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1:47:10bbbyan Q: Do you have any project recommendations for beginner-intermediate C programmers?

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1:47:46ivereadthesequel Q: Seems a bit casual today, what's your favorite album?

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1:47:46ivereadthesequel Q: Seems a bit casual today, what's your favorite album?

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1:47:46ivereadthesequel Q: Seems a bit casual today, what's your favorite album?

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1:49:54jessef Q: What is the craziest file format to parse, in your opinion?

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1:49:54jessef Q: What is the craziest file format to parse, in your opinion?

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1:49:54jessef Q: What is the craziest file format to parse, in your opinion?

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1:50:54vaualbus Q: What is the most difficult file format to read? Have you ever try to read ttf files? And try doing a vector renderer for those in OpenGL?

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1:50:54vaualbus Q: What is the most difficult file format to read? Have you ever try to read ttf files? And try doing a vector renderer for those in OpenGL?

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1:50:54vaualbus Q: What is the most difficult file format to read? Have you ever try to read ttf files? And try doing a vector renderer for those in OpenGL?

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1:51:08frostyninja Q: What's your favourite pasta dish?

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1:51:08frostyninja Q: What's your favourite pasta dish?

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1:51:08frostyninja Q: What's your favourite pasta dish?

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1:51:41jacksonbanan Q: How come no one has made a solid debugger? Is it difficult?

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1:51:41jacksonbanan Q: How come no one has made a solid debugger? Is it difficult?

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1:51:41jacksonbanan Q: How come no one has made a solid debugger? Is it difficult?

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1:52:34runamar Q: Will the game be only room based?

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1:52:34runamar Q: Will the game be only room based?

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1:52:34runamar Q: Will the game be only room based?

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1:52:43quickshift_ Q: Bit off topic, but are you a musician? Didn't you write some music for Handmade Hero yourself?

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1:52:43quickshift_ Q: Bit off topic, but are you a musician? Didn't you write some music for Handmade Hero yourself?

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1:52:43quickshift_ Q: Bit off topic, but are you a musician? Didn't you write some music for Handmade Hero yourself?

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1:52:54tavqua Q: This may be too in-depth of a question, but: how can I send data over the internet for a game that can be picked up in a code that I write

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1:52:54tavqua Q: This may be too in-depth of a question, but: how can I send data over the internet for a game that can be picked up in a code that I write

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1:52:54tavqua Q: This may be too in-depth of a question, but: how can I send data over the internet for a game that can be picked up in a code that I write

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1:53:18thejimjames40 Q: Will we be parsing compiler debug output (DWARF?) for cool in-game debug stuff?

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1:53:18thejimjames40 Q: Will we be parsing compiler debug output (DWARF?) for cool in-game debug stuff?

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1:53:18thejimjames40 Q: Will we be parsing compiler debug output (DWARF?) for cool in-game debug stuff?

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1:53:27enemymouse Q: 3ds is worse than psd?

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1:53:27enemymouse Q: 3ds is worse than psd?

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1:53:27enemymouse Q: 3ds is worse than psd?

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1:53:47bbbyan Q: We've had a Handmade Ray bonus series, what about Handmade Asteroids?

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1:53:47bbbyan Q: We've had a Handmade Ray bonus series, what about Handmade Asteroids?

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1:53:47bbbyan Q: We've had a Handmade Ray bonus series, what about Handmade Asteroids?

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1:53:52Call it a day

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1:53:52Call it a day

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1:53:52Call it a day

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