Texture2D.LoadImage color/colour problems

Question

I'm doing some version of palette swapping for sprites, and I keep $$anonymous$$tting a problem. Unity seems to be altering the colours in PNG files when importing.

As you can see below, when the sprite is saved it has very specific values. when I run the code below to immediately save the png after loading, I get different colours.

If anyone has any idea about what I'm doing wrong here, I'd love to know. To avoid any arguments: I'm not here for a discussion about whether t$$anonymous$$s is the best way to do t$$anonymous$$s, I'm after an answer to a weird problem.

alt text

 byte[] spriteBytes = File.ReadAllBytes(fullPath);

 Texture2D temp = new Texture2D(2, 2, TextureFormat.RGBA32,false);
 //to get the proper size
 if (!temp.LoadImage(spriteBytes)) {
   Debug.LogError("image file:[" + fullPath + "] could not be read.");
   return;
 }
 //import again at the correct size (apparently t$$anonymous$$s is a t$$anonymous$$ng).
 spriteSheetTexture = new Texture2D(temp.width, temp.height, TextureFormat.RGBA32,false);
 
 if (spriteSheetTexture.LoadImage(spriteBytes))  {
   File.WriteAllBytes("a path dont worry about it.png", spriteSheetTexture.EncodeToPNG());
 }

unity version info: 2019.1.7f1personal Visual studio: 2017 15.9.12 community

Answer 1

Best Answer

Answer by Bunny83 · Jun 19, 2019 at 01:33 AM

Well, I just did a few tests with LoadImage and EncodePNG. It looks like Unity has issues with loading png files that contain gamma information. When I create a 4k texture with all 16 million colors in Unity and export as png, the resulting file does not contain any of the usual png chunks. It only contains the IHDR as well as several IDAT chunk with the data. If I load t$$anonymous$$s png dynamically through LoadImage, everyt$$anonymous$$ng is right.

However when I open that png in mspaint and re-save it, the new png will have a sRGB, gAMA and pHYs chunk as usual. When we load t$$anonymous$$s at runtime we get errors in the colors that is 0 at black (0) ramps up to "-9" at 128 and ramps back down to 0 as we go further up to 255. So it seems to be a wrong gamma correction that is going on since gamma correction is a power function. That's why the error is largest at 128 ( 0.5 ) and 0 at 0 (0.0) and 255 (1.0).

I would recommend to file a bug report

edit

I just tried using my PNGTools w$$anonymous$$ch I wrote for t$$anonymous$$s question over here and removed the sRGB, gAMA and pHYs chunks before letting Unity load the png and it seems to work with my test image.

Specifically instead of

 byte[] spriteBytes = File.ReadAllBytes(fullPath);

do t$$anonymous$$s:

     byte[] spriteBytes;
     using (var fileStream = System.IO.File.OpenRead("C:\\Data\\ColorTest.png"))
     using (var reader = new System.IO.BinaryReader(fileStream))
     {
         var png = PNGTools.ReadPNGFile(reader);
         for (int i = png.chunks.Count - 1; i >= 0; i--)
         {
             var c = png.chunks[i];
             if (c.type == EPNGChunkType.gAMA ||
                 c.type == EPNGChunkType.pHYs ||
                 c.type == EPNGChunkType.sRBG)
             {
                 png.chunks.RemoveAt(i);
             }
         }
         using (var mem = new MemoryStream())
         {
             PNGTools.WritePNGFile(png, new BinaryWriter(mem));
             spriteBytes = mem.ToArray();
         }
     }

Note that t$$anonymous$$s is not guaranteed to work with every kind of png file.

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Bunny83 · Jun 19, 2019 at 01:34 AM 1

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Note that when importing the png actually as asset through the Unity editor the colors are correct. So it seems to be an issue in the runtime png loader.

sam-rae · Jun 19, 2019 at 08:43 AM 0

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You absolute legend. I love the "I have a problem so I built a tool to fix it" approach.

I ended up "solving" it at 1am by reading the data in via the System.Drawing.Bitmap class and then using

    Texture2D.LoadRawTextureData(byte[] data);

Interesting note: The bitmap class seems to load things upside-down but you can then flip it

sam-rae · Jun 19, 2019 at 08:45 AM 0

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Looks like it's on purpose:

issue

raarc · Jun 21, 2020 at 03:13 AM 0

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@bunny83 You saved me with this code, thank you so much. I had to change some things, I switched EPNGChunkType to EChunkType and the gAMA uint value was different on my files so I changed it to 1766015824. It works perfectly thanks a lot for this tool

Bunny83 raarc · Jun 21, 2020 at 03:20 AM 0

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Uhm the value you just mentioned is not a gAMA chunk. It's an iCCP chunk. The value 1766015824 is 0x69434350 in hex which literally represents the ascii characters "iCCP". All PNG chunk headers are 4 ascii characters. The iCCP chunk is an embedded color profile. So yes, it could also cause some color "errors".

raarc Bunny83 · Jun 21, 2020 at 09:12 AM 0

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hmm then my file did not have a gAMA chunk. Your tool detected 6 chunks and 5 of them were in your enum EChunkType and the one that didnt print was gAMA, your tool told me the id of the one unnacouted for was 1766015824 so i replaced it on gAMA and it fixed the problem. Before I replaced that value the colors were still returning wrong so that chunk was definitely causing the problem.(maybe in conjunction with phys and srgb). Your tool is amazing by the way! Thanks again

emongev · Aug 22, 2020 at 08:54 AM 0

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Thank you so much! you saved me with this, the logic behind was wrong was way over my head, but luckily your fix solved it! :)

Texture2D.LoadImage color/colour problems

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