Re-saving 16bit PNG image with straight alpha multiple times corrupts the image #99701

New Issue

quvr · 2022-07-14T15:07:56+02:00

quvr commented

2022-07-14 15:07:56 +02:00

System Information
Operating system: Windows 10
Graphics card: AMD

Blender Version
Broken: 3.2.1
Worked: ?

Short description of error

Re-saving 16bit PNG image with straight alpha multiple times corrupts the image.
As far as I can tell it's broken only for 16/32bit images and works for 8bit

Exact steps for others to reproduce the error
Texture.zip

Open the file, paint something half-transparent (optionally)
Press "Save image"
Press "Reload image"
Image should look different than before saving

**System Information** Operating system: Windows 10 Graphics card: AMD **Blender Version** Broken: 3.2.1 Worked: ? **Short description of error** Re-saving 16bit PNG image with straight alpha multiple times corrupts the image. As far as I can tell it's broken only for 16/32bit images and works for 8bit **Exact steps for others to reproduce the error** [Texture.zip](https://archive.blender.org/developer/F13292406/Texture.zip) 1) Open the file, paint something half-transparent (optionally) 2) Press "Save image" 3) Press "Reload image" 4) Image should look different than before saving

quvr commented

2022-07-14 15:07:56 +02:00

Added subscriber: @Quver

Anton Raves commented

2022-07-15 16:18:42 +02:00

Added subscriber: @Memento

Omar Emara commented

2022-07-18 09:51:04 +02:00

Added subscriber: @OmarEmaraDev

Omar Emara commented

2022-07-18 09:51:04 +02:00

Changed status from 'Needs Triage' to: 'Needs User Info'

Omar Emara commented

2022-07-18 09:51:04 +02:00

Testing a 32-bit OpenEXR image, it seems the difference is just alpha premultiplication, which seems to the expected behavior. Is this the difference you are experiencing or is there something else?

quvr commented

2022-07-18 12:30:31 +02:00

@OmarEmaraDev you're right the issue is only with alpha channel, colors seems intact. I'm sorry I did not specify a format to export because my blender is configured to save as 16bit png RGBA by default (I think?) so I assumed your export will be in png too. No need to choose any other format to reproduce this. Just open the file and press alt+s, alt+r multiple times. The original file is 16bit png RGBA as well so I expect it to look the same without any changes, but in result the alpha channel seems wrong as you mentioned. Same thing when export to exr.

I think this is still a bug.
If you save the file as 8bit png there are no issues. If I paint something on that texture and want to export it back there are no ways to do this without converting it to 8bit so it looks the same as in blender. Or at least I couldn't find a way to do so. Maybe I'm missing something?

A video just in case
1.mp4

@OmarEmaraDev you're right the issue is only with alpha channel, colors seems intact. I'm sorry I did not specify a format to export because my blender is configured to save as 16bit png RGBA by default (I think?) so I assumed your export will be in png too. No need to choose any other format to reproduce this. Just open the file and press alt+s, alt+r multiple times. The original file is 16bit png RGBA as well so I expect it to look the same without any changes, but in result the alpha channel seems wrong as you mentioned. Same thing when export to exr. I think this is still a bug. If you save the file as 8bit png there are no issues. If I paint something on that texture and want to export it back there are no ways to do this without converting it to 8bit so it looks the same as in blender. Or at least I couldn't find a way to do so. Maybe I'm missing something? A video just in case [1.mp4](https://archive.blender.org/developer/F13298407/1.mp4)

Omar Emara commented

2022-07-18 16:53:43 +02:00

Changed status from 'Needs User Info' to: 'Needs Developer To Reproduce'

Omar Emara commented

2022-07-18 16:53:43 +02:00

I see. You mentioned 32bit so I assumed you were talking about EXR images.
I can reproduce the issue, and it doesn't seem to be normal alpha premultiplication and I can see the option being set to Straight Alpha.
But I also don't understand the behavior, so tagging the module for more information.
Might be related to #99186.

I see. You mentioned 32bit so I assumed you were talking about EXR images. I can reproduce the issue, and it doesn't seem to be normal alpha premultiplication and I can see the option being set to Straight Alpha. But I also don't understand the behavior, so tagging the module for more information. Might be related to #99186.

Omar Emara changed title from ~~"Save image" weird color management~~ to Re-saving 16bit PNG image with straight alpha multiple times corrupts the image

2022-07-18 16:55:04 +02:00

Vyacheslav Kobozev commented

2022-11-05 21:15:55 +01:00

Added subscriber: @Vyach

Vyacheslav Kobozev commented

2022-11-05 21:15:55 +01:00

This comment was removed by @Vyach

*This comment was removed by @Vyach*

Vyacheslav Kobozev commented

2022-11-05 21:20:57 +01:00

This comment was removed by @Vyach

*This comment was removed by @Vyach*

Michael Soluyanov commented

2022-11-05 21:45:08 +01:00

Added subscriber: @crantisz

Michael Soluyanov commented

2022-11-05 21:45:08 +01:00

This is not a color management issue. This is an Issue related to alpha pre-multiplication.

Here is the case. I saved one render in EXR PNG and PNG16.

EXR is good for image compositing, so that we know that it has pre-multiplied alpha. If we load it into compositor, it shows correct results, and we can note that alpha setting in image node is premultiplied by default.

But we can say what image is straight, so that Blender multiply RGB channels by alpha:

PNG16 we know has straight alpha. Blender indicates that and set alpha setting to straight by default. So that we lost glowing effect, as expected.

We can force image to be pre-multiplied by switching the setting. And in this case we can see the white border around cubes as an artifact of, shall we say, "de-premultiplication" as expected:

Now load PNG8. It loads as premultiplied even alpha setting is straight by default:

So we know that PNG8 has straight alpha. And Blender shows it as straight alpha correctly. But what is incorrect, the image itself. In fact, Blender makes "de-premultiplication" under the hood and PNG8 with alpha set to straight displays as pre-multiplied. And we can be convinced of that by changing alpha to premultiplied revealing double "de-premultiplication" (One from loading image and one because of PNG specification) or double dividing original pixels by alpha:

So in order to get correct representation of PNG8 we need to add additional alpha convert step:

So looks like this hidden "de-premultiplication" process is made for image texturing. I don't know that's happening under the hood, but it looks like that in texturing PNG's functions as straight, even then the whole "pipeline" uses pre-multiplied. So somewhere in the past someone decided to make PNG8 for texturing and PNG16 for compositing by doing this weird thing under the hood, and making this headache for future us, so that we struggle with different PNG's in one application.

Images:

This is not a color management issue. This is an Issue related to alpha pre-multiplication. Here is the case. I saved one render in EXR PNG and PNG16. EXR is good for image compositing, so that we know that it has pre-multiplied alpha. If we load it into compositor, it shows correct results, and we can note that alpha setting in image node is premultiplied by default. ![image.png](https://archive.blender.org/developer/F13859779/image.png) But we can say what image is straight, so that Blender multiply RGB channels by alpha: ![image.png](https://archive.blender.org/developer/F13859783/image.png) PNG16 we know has straight alpha. Blender indicates that and set alpha setting to straight by default. So that we lost glowing effect, as expected. ![image.png](https://archive.blender.org/developer/F13859793/image.png) We can force image to be pre-multiplied by switching the setting. And in this case we can see the white border around cubes as an artifact of, shall we say, "de-premultiplication" as expected: ![image.png](https://archive.blender.org/developer/F13859799/image.png) Now load PNG8. It loads as premultiplied even alpha setting is straight by default: ![image.png](https://archive.blender.org/developer/F13859807/image.png) So we know that PNG8 has straight alpha. And Blender shows it as straight alpha correctly. But what is incorrect, the image itself. In fact, Blender makes "de-premultiplication" under the hood and PNG8 with alpha set to straight displays as pre-multiplied. And we can be convinced of that by changing alpha to premultiplied revealing double "de-premultiplication" (One from loading image and one because of PNG specification) or double dividing original pixels by alpha: ![image.png](https://archive.blender.org/developer/F13859822/image.png) So in order to get correct representation of PNG8 we need to add additional alpha convert step: ![image.png](https://archive.blender.org/developer/F13859826/image.png) So looks like this hidden "de-premultiplication" process is made for image texturing. I don't know that's happening under the hood, but it looks like that in texturing PNG's functions as straight, even then the whole "pipeline" uses pre-multiplied. So somewhere in the past someone decided to make PNG8 for texturing and PNG16 for compositing by doing this weird thing under the hood, and making this headache for future us, so that we struggle with different PNG's in one application. Images: ![test.png](https://archive.blender.org/developer/F13859844/test.png) ![test16.png](https://archive.blender.org/developer/F13859845/test16.png) ![test.exr](https://archive.blender.org/developer/F13859842/test.exr)

Vyacheslav Kobozev commented

2022-11-05 21:56:57 +01:00

image with straight alpha, it have very different alpha and color channel, but it loaded as premultiplied
png8 wrong reading.blend

image with straight alpha, it have very different alpha and color channel, but it loaded as premultiplied [png8 wrong reading.blend](https://archive.blender.org/developer/F13859866/png8_wrong_reading.blend)

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Re-saving 16bit PNG image with straight alpha multiple times corrupts the image #99701