Flame is transparent together with the background #110607

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opened 2023-07-29 11:59:28 +02:00 by Shoya-Kudo · 3 comments

Shoya-Kudo commented

2023-07-29 11:59:28 +02:00

System Information
Operating system:Widows11 home 64bit
Graphics card:12th gen intel(r) core(tm) i7-1260p 2.10 ghz

Blender Version
Broken: 3.6

Short description of error
When I used Cycles and tried to render the flame as a background transparent material in PNG format RGBA, the flame as well as the background were transparent together. Please help.

Exact steps for others to reproduce the error
Based on the default startup or an attached .blend file (as simple as possible).

**System Information** Operating system:Widows11 home 64bit Graphics card:12th gen intel(r) core(tm) i7-1260p 2.10 ghz **Blender Version** Broken: 3.6 **Short description of error** When I used Cycles and tried to render the flame as a background transparent material in PNG format RGBA, the flame as well as the background were transparent together. Please help. **Exact steps for others to reproduce the error** Based on the default startup or an attached .blend file (as simple as possible).

blue-fire-small.blend

1006 KiB

Shoya-Kudo added the

labels 2023-07-29 11:59:28 +02:00

YimingWu commented

2023-07-29 15:28:42 +02:00

Member

Hi! looks like the flame is rendered correctly here both in 3.6 and 4.0 (I cleared bake and re-baked 1 frame).

The file itself opens with a baked cache path /AppData/Local/Temp/blender_a03632/cache_fluid_4b85f272/, could you go to your user AppData folder and check if this cache exists?

Hi! looks like the flame is rendered correctly here both in 3.6 and 4.0 (I cleared bake and re-baked 1 frame). The file itself opens with a baked cache path `/AppData/Local/Temp/blender_a03632/cache_fluid_4b85f272/`, could you go to your user AppData folder and check if this cache exists? ![图片](/attachments/cc3b411d-c80d-450a-844a-e359c51258dd)

107 KiB

YimingWu added

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labels 2023-07-29 15:29:15 +02:00

YimingWu changed title from ~~About transparent background~~ to Flame is transparent together with the background

2023-07-29 15:29:40 +02:00

Shoya-Kudo commented

2023-07-29 15:39:44 +02:00

Author

Thanks for the reply. I just checked and the cache itself is there. When I do the background transparency, I can render it. But when I download it and open it as a PNG, the flame as well as the background is gone.

Thanks for the reply. I just checked and the cache itself is there. When I do the background transparency, I can render it. But when I download it and open it as a PNG, the flame as well as the background is gone.

スクリーンショット 2023-07-29 223546.png

398 KiB

スクリーンショット 2023-07-29 223546.png

Evan Wilson commented

2023-07-30 05:45:54 +02:00

Member

This is actually a limitation of PNG files themselves. Blender's render engines, including Cycles, output associated (premultiplied) alpha. The PNG specification states that PNG files use unassociated (non-premultiplied) alpha, and that associated (premultiplied) alpha must be converted.
https://www.w3.org/TR/png/#6AlphaRepresentation

The PNG specification says

If the alpha value is zero, output black (zeroes).

when converting premultiplied alpha to PNG's non-premultiplied format.
https://www.w3.org/TR/png/#12Alpha-channel-creation

It is recommended to use a file format that supports associated (premultiplied), such as OpenEXR, instead.

As this is a file format limitation, and not a bug, I'll go ahead and close this report.

This is actually a limitation of PNG files themselves. Blender's render engines, including Cycles, output associated (premultiplied) alpha. The PNG specification states that PNG files use unassociated (non-premultiplied) alpha, and that associated (premultiplied) alpha must be converted. https://www.w3.org/TR/png/#6AlphaRepresentation The PNG specification says > If the alpha value is zero, output black (zeroes). when converting premultiplied alpha to PNG's non-premultiplied format. https://www.w3.org/TR/png/#12Alpha-channel-creation It is recommended to use a file format that supports associated (premultiplied), such as OpenEXR, instead. As this is a file format limitation, and not a bug, I'll go ahead and close this report.

Evan Wilson closed this issue

2023-07-30 05:45:56 +02:00

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