16bitPNG breaks after Packing and editing #108866

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Velland1982 · 2023-06-11T20:48:26+02:00

Velland1982 commented

2023-06-11 20:48:26 +02:00

System Information
Operating system:Win10/Win10
Graphics card: RTX 3090/RTX2080

Blender Version
Broken:3.4/3.5
Worked: ?

I Created an albedo map based on a 16 bit PNG Texture, by texture painting.
I packed the Image for work on the move.
After reloading the Bledn file the Image got darker.

Reproduce Based on the default startup:
Pack a 16 bit PNG Image.
modify it with blender internal tools.
Save Image
Save Blend
Reload

The Images below show the Original and just 2 edit, save and reloads

**System Information** Operating system:Win10/Win10 Graphics card: RTX 3090/RTX2080 **Blender Version** Broken:3.4/3.5 Worked: ? I Created an albedo map based on a 16 bit PNG Texture, by texture painting. I packed the Image for work on the move. After reloading the Bledn file the Image got darker. Reproduce Based on the default startup: Pack a 16 bit PNG Image. modify it with blender internal tools. Save Image Save Blend Reload The Images below show the Original and just 2 edit, save and reloads

step 3.PNG

98 KiB

step 1.PNG

132 KiB

png_16bit_bug.blend

64 MiB

step 2.PNG

120 KiB

Velland1982 added the

labels 2023-06-11 20:48:27 +02:00

YimingWu commented

2023-06-12 06:04:47 +02:00

This could be very likely due to wrong srgb conversion... Will take a look.

YimingWu commented

2023-06-12 07:17:40 +02:00

Can confirm this being an sRGB problem, where the file is saved as linear and read & displayed as sRGB.

Note that PNG specification never supported 16F/32F pixel format, it only supports up to 16I. Not sure what's happening here, likely a interface ambiguity where it shows the buffer format as 16F/32F but saves as integer. PNG R/W has a special sRGB tag and it should only apply to 8 bit images (http://www.libpng.org/pub/png/libpng-manual.txt).

I'm unable to extract the original png to inspect due to the very fact that it doesn't technically support 16F/32F, when unpacking the image, blender automatically uses exr format. @Velland1982 Do you have the original texture png file before packing?

From the look of it, it's likely that somehow the PNG is read to be assumed as SRGB while it's storing linear values, and a display transformation made it even darker.

By unpacking the file into an exr and repacking it, blender writes and reads the image correctly regardless of color space choices. So the problem is mainly about how this png ended up like this in the first place...

Can confirm this being an sRGB problem, where the file is saved as linear and read & displayed as sRGB. Note that PNG specification never supported 16F/32F pixel format, it only supports up to 16I. Not sure what's happening here, likely a interface ambiguity where it shows the buffer format as 16F/32F but saves as integer. PNG R/W has a special sRGB tag and it `should` only apply to 8 bit images (http://www.libpng.org/pub/png/libpng-manual.txt). I'm unable to extract the original png to inspect due to the very fact that it doesn't technically support 16F/32F, when unpacking the image, blender automatically uses exr format. @Velland1982 Do you have the original texture `png` file before packing? From the look of it, it's likely that somehow the PNG is read to be assumed as SRGB while it's storing linear values, and a display transformation made it even darker. By unpacking the file into an exr and repacking it, blender writes and reads the image correctly regardless of color space choices. So the problem is mainly about how this png ended up like this in the first place...

YimingWu added

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Needs Information from User

and removed

Status

Needs Triage

labels 2023-06-12 07:23:54 +02:00

Velland1982 commented

2023-06-12 18:16:33 +02:00

Here the original PNG.
I used GIMP to rescale it from 4096² to 2048² and then just loaded into the blend and started blocking out the areas in texture paint.
I always pack my external files to keep the projekt in one nice package.

Here the original PNG. I used GIMP to rescale it from 4096² to 2048² and then just loaded into the blend and started blocking out the areas in texture paint. I always pack my external files to keep the projekt in one nice package.

Velland1982 commented

2023-06-12 18:16:35 +02:00

Here the original PNG.
I used GIMP to rescale it from 4096² to 2048² and then just loaded into the blend and started blocking out the areas in texture paint.
I always pack my external files to keep the projekt in one nice package.

Here the original PNG. I used GIMP to rescale it from 4096² to 2048² and then just loaded into the blend and started blocking out the areas in texture paint. I always pack my external files to keep the projekt in one nice package.

Body_diffuse.png

628 KiB

Velland1982 closed this issue

2023-06-12 18:16:35 +02:00

Blender Bot added

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Archived

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labels 2023-06-12 18:16:36 +02:00

Velland1982 reopened this issue

2023-06-12 18:16:41 +02:00

Blender Bot added

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labels 2023-06-12 18:16:43 +02:00

YimingWu commented

2023-06-13 08:43:48 +02:00

Hi @Velland1982 , looks like GIMP here saved this as 16bit sRGB file, which is technically not a combination that matches PNG specification. It's actually a known issue with GIMP. You should manually change the image from sRGB to Linear in GIMP whenever you want to save a png that's not 8 bit.

identify -verbose Body_diffuse.png shows both sRGB rendering intent tag and icc profile, which is not generally used together, and can confuse some software.

  Rendering intent: Perceptual
  Gamma: 0.454545
...
  icc:model: sRGB
  png:bKGD: chunk was found (see Background color, above)
  png:iCCP: chunk was found

Will close the issue for now since it's not a bug with blender.

Hi @Velland1982 , looks like GIMP here saved this as 16bit sRGB file, which is technically not a combination that matches PNG specification. It's actually a known issue with GIMP. You should manually change the image from sRGB to Linear in GIMP whenever you want to save a png that's not 8 bit. `identify -verbose Body_diffuse.png` shows both sRGB rendering intent tag and icc profile, which is not generally used together, and can confuse some software. ``` Rendering intent: Perceptual Gamma: 0.454545 ... icc:model: sRGB png:bKGD: chunk was found (see Background color, above) png:iCCP: chunk was found ``` Will close the issue for now since it's not a bug with blender.

YimingWu closed this issue

2023-06-13 08:43:54 +02:00

Blender Bot added

and removed

labels 2023-06-13 08:43:55 +02:00

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