Combine Color / Set Alpha Nodes Incorrectly Handling Alpha Channel #127091

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Alex-R-Heskett · 2024-09-03T03:34:59+02:00

Alex-R-Heskett commented

2024-09-03 03:34:59 +02:00

System Information
Operating system: Windows
Graphics card: Nvidia 3070 Laptop GPU

Blender Version
Broken: 4.2.0
Worked: Unclear, found to be present as far back as 3.4

Short description of error
When storing an image in the alpha channel of a texture via the combine color or set alpha (when set to Replace Alpha) nodes, the image is saved incorrectly with the alpha channel affecting the other channels. Rendering a transparent material using the same texture for transparency does not result in these same artifacts. Upon closer inspection, Blender seems to be dividing the red, green, and blue channels by the value of the alpha channel.

In the demo scene, the following two gradients are rendered on a plane:

In the compositor, the second image is placed in the alpha channel of the first before it is saved. When the resulting file is reopened on the plane, the alpha channel interferes with the other channels like so:

The same artifacts are visible when inspecting the channels of the texture in photoshop. It is unclear why this is happening, and it occurs even when the texture node is set to CHANNEL PACKED

Exact steps for others to reproduce the error
Open the .blend
Render an image
Open the image in the image texture node in the plane material to view artifacts

**System Information** Operating system: Windows Graphics card: Nvidia 3070 Laptop GPU **Blender Version** Broken: 4.2.0 Worked: Unclear, found to be present as far back as 3.4 **Short description of error** When storing an image in the alpha channel of a texture via the combine color or set alpha (when set to Replace Alpha) nodes, the image is saved incorrectly with the alpha channel affecting the other channels. Rendering a transparent material using the same texture for transparency does not result in these same artifacts. Upon closer inspection, Blender seems to be dividing the red, green, and blue channels by the value of the alpha channel. In the demo scene, the following two gradients are rendered on a plane: <img width="497" alt="image" src="attachments/7739b8a5-e689-4c8e-a7e0-0fdfec16f143"> <img width="499" alt="image" src="attachments/c069b4c1-69da-4693-b879-a99238d3b31f"> In the compositor, the second image is placed in the alpha channel of the first before it is saved. When the resulting file is reopened on the plane, the alpha channel interferes with the other channels like so: <img width="494" alt="image" src="attachments/db3666cf-b4f4-40ed-91a1-87dfc965b765"> The same artifacts are visible when inspecting the channels of the texture in photoshop. It is unclear why this is happening, and it occurs even when the texture node is set to CHANNEL PACKED **Exact steps for others to reproduce the error** Open the .blend Render an image Open the image in the image texture node in the plane material to view artifacts

alpha test.blend

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image.png

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image.png

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image.png

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Alex-R-Heskett added the

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Normal

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