Transform Node Messes Up Alpha Channel Manipulation #57568

New Issue

Thomas Downs · 2018-11-01T19:08:12+01:00

Thomas Downs commented

2018-11-01 19:08:12 +01:00

System Information
Arch Linux
Nvidia 750ti

Blender Version
Broken: 2.79 (Tried 2.8 as well and I have the same issue)

As referenced here:
https://blenderartists.org/t/filters-unintentionally-crop-transformed-image/1132477/4

When I take an image in the compositor and scale it up 2x or whatever and then separate the RGBA and dilate/erode the alpha and use that to set the alpha on the image, it only works within the size of the original image and not the scaled up image. Also even if I use the R, G, or B, it still only uses the size of the original image and not that of the scaled up one.

Cropped if I try to mess with the alpha after a transform.

Uncropped if I mute my changes to the alpha.

Also I believe that if you scale down

dilatecropissue.blend

**System Information** Arch Linux Nvidia 750ti **Blender Version** Broken: 2.79 (Tried 2.8 as well and I have the same issue) As referenced here: https://blenderartists.org/t/filters-unintentionally-crop-transformed-image/1132477/4 When I take an image in the compositor and scale it up 2x or whatever and then separate the RGBA and dilate/erode the alpha and use that to set the alpha on the image, it only works within the size of the original image and not the scaled up image. Also even if I use the R, G, or B, it still only uses the size of the original image and not that of the scaled up one. ![problem1.png](https://archive.blender.org/developer/F5327471/problem1.png) Cropped if I try to mess with the alpha after a transform. ![problem2.png](https://archive.blender.org/developer/F5327470/problem2.png) Uncropped if I mute my changes to the alpha. ![problem3.png](https://archive.blender.org/developer/F5327469/problem3.png) Also I believe that if you scale down [dilatecropissue.blend](https://archive.blender.org/developer/F5327448/dilatecropissue.blend)

Thomas Downs commented

2018-11-01 19:08:12 +01:00

Added subscriber: @tnelsond

Philipp Oeser commented

2018-11-07 15:04:46 +01:00

Added subscribers: @Jeroen-Bakker, @brecht, @lichtwerk

Philipp Oeser commented

2018-11-07 15:04:46 +01:00

I guess this is known limitation of the compositor (but I am not sure).
Workaround is (as you said) to first do the erosion first, then the scaling.

@Jeroen-Bakker, @brecht : could you comment? [not sure if we can close this as a "known limitation"?]

I guess this is known limitation of the compositor (but I am not sure). Workaround is (as you said) to first do the erosion first, then the scaling. @Jeroen-Bakker, @brecht : could you comment? [not sure if we can close this as a "known limitation"?]

Thomas Downs commented

2018-11-07 16:16:26 +01:00

Working around it would be easier if the dilate/erode node had an input for step value.

Germano Cavalcante commented

2020-01-27 23:22:41 +01:00

Added subscriber: @mano-wii

Germano Cavalcante commented

2020-01-27 23:22:41 +01:00

This seems to be a problem specific of the Dilate/Erode node.
I think it deserves an investigation before it is considered a Known Issue.
But I fear that no one will work on it for the upcoming 6 months.

This seems to be a problem specific of the `Dilate/Erode` node. I think it deserves an investigation before it is considered a `Known Issue`. But I fear that no one will work on it for the upcoming 6 months.

Manuel Castilla commented

2020-10-18 14:13:24 +02:00

Added subscriber: @manzanilla

Manuel Castilla commented

2020-10-18 14:13:24 +02:00

I've seen the compositor implementation and seems to me that this behaviour is by design.
When you scale up, the resolution keeps being the same, it just scales the image within the resolution canvas and that's why it gets cropped.
The confusion comes from the fact that dilate/erode is buffered and the other nodes are not.

If you scale up before dilate/erode (or any buffered node), scaling takes effect with the input image resolution after this node finishes and image is cropped (in this case the applied alpha).
If you do scaling after dilate/erode, scaling will be done within the composite resolution which is the output resolution, which in this case is bigger than the input image resolution and that's why it doesn't appear cropped.

I agree this behaviour is very confusing for the user. If all operations were buffered it would just always get cropped which is what should happen taken into account that scaling doesn't resize resolution canvas.
To avoid cropping when scaling up, there could be an option to allow the user to resize the canvas too, but to give the user the power to increase resolution would mean that he can make the compositor hang easily with huge resolutions.

But all this is a compositor design decision.

I've seen the compositor implementation and seems to me that this behaviour is by design. When you scale up, the resolution keeps being the same, it just scales the image within the resolution canvas and that's why it gets cropped. The confusion comes from the fact that dilate/erode is buffered and the other nodes are not. If you scale up before dilate/erode (or any buffered node), scaling takes effect with the input image resolution after this node finishes and image is cropped (in this case the applied alpha). If you do scaling after dilate/erode, scaling will be done within the composite resolution which is the output resolution, which in this case is bigger than the input image resolution and that's why it doesn't appear cropped. I agree this behaviour is very confusing for the user. If all operations were buffered it would just always get cropped which is what should happen taken into account that scaling doesn't resize resolution canvas. To avoid cropping when scaling up, there could be an option to allow the user to resize the canvas too, but to give the user the power to increase resolution would mean that he can make the compositor hang easily with huge resolutions. But all this is a compositor design decision.

Philipp Oeser removed the

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Transform Node Messes Up Alpha Channel Manipulation #57568