Bug 420013

Created attachment 127487 [details]
Example image of artifacts

SUMMARY
This happens for versions 4.2.0 onwards but does not happen in version 4.1.7.

If a simple painted shape has any blur filter applied then artifacts are created in the RGB content and these can be revealed by splitting the alpha out into a mask.

A .png file is attached (blur-artifacts-revealed.png) that shows an example.

Here is a .kra file with further examples for convenience of investigation:
https://www.dropbox.com/s/8ipsb35hsrgeet7/Sample-Blur-Artifacts.kra?dl=1
This was made using the 5.0.0 pre-alpha appimage 08-Apr git 78d1d45

STEPS TO REPRODUCE
1. On a new paint layer, paint a simple shape then apply any blur filter.
2. Do Layer - Split - Split Alpha Into Mask on that layer
3. Turn off the mask to reveal the artifacts.
4. Note that trying this with a filled rectangle of any size or aspect ratio does not give artifacts.
5. Note the 'tile' structure of the artifacts and that this is not 512 x 512 and seems to vary in size.
6. Note that varying the type of blur filter or the parameters of the blur filter does not make significant changes to the nature of the artifacts.
7. Having revealed the artifacts by alpha splitting, flatten the layer to combine the alpha with the image then reapply a split alpha into mask.
Note that the artifacts are now gone. (Rendered away by the flattening?).
8. After the blur filter is applied to a layer, turn off the background layer and export the layer as a .png with alpha preserved. Then open that .png file and split the alpha out to a mask.
Note that there are no artifacts.

OBSERVED RESULT
See Steps To reproduce

EXPECTED RESULT
No artifacts

SOFTWARE/OS VERSIONS

Krita

 Version: 5.0.0-prealpha (git 78d1d45)
 Languages: en_GB, en, en, en_GB, en
 Hidpi: true

Qt

  Version (compiled): 5.12.7
  Version (loaded): 5.12.7

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-8-amd64
  Pretty Productname: Debian GNU/Linux 10 (buster)
  Product Type: debian
  Product Version: 10
  Desktop: MATE

OpenGL Info
 
  Vendor:  "NVIDIA Corporation" 
  Renderer:  "GeForce GTX 750 Ti/PCIe/SSE2" 
  Version:  "4.6.0 NVIDIA 418.74" 
  Shading language:  "4.60 NVIDIA" 
  Requested format:  QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 0, colorSpace QSurfaceFormat::DefaultColorSpace, profile  QSurfaceFormat::CompatibilityProfile) 
  Current format:    QSurfaceFormat(version 4.6, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 0, colorSpace QSurfaceFormat::DefaultColorSpace, profile  QSurfaceFormat::CompatibilityProfile) 
     Version: 4.6
     Supports deprecated functions true 
     is OpenGL ES: false 

QPA OpenGL Detection Info 
  supportsDesktopGL: true 
  supportsOpenGLES: true 
  isQtPreferOpenGLES: false 

Hardware Information

  GPU Acceleration: auto
  Memory: 16039 Mb
  Number of Cores: 8
  Swap Location: /tmp

Current Settings

  Current Swap Location: /tmp
  Current Swap Location writable: true
  Undo Enabled: true
  Undo Stack Limit: 18
  Use OpenGL: true
  Use OpenGL Texture Buffer: true
  Use AMD Vectorization Workaround: false
  Canvas State: OPENGL_SUCCESS
  Autosave Interval: 60
  Use Backup Files: true
  Number of Backups Kept: 1
  Backup File Suffix: ~
  Backup Location: Same Folder as the File
  Backup Location writable: false
  Use Win8 Pointer Input: false
  Use RightMiddleTabletButton Workaround: false
  Levels of Detail Enabled: false
  Use Zip64: false


Display Information
Number of screens: 2
	Screen: 0
		Name: DVI-D-1
		Depth: 24
		Scale: 1
		Resolution in pixels: 1280x1024
		Manufacturer: Dell Inc.
		Model: DELL 1704FPV-
		Refresh Rate: 60
	Screen: 1
		Name: DVI-D-0
		Depth: 24
		Scale: 1
		Resolution in pixels: 1280x1024
		Manufacturer: Dell Inc.
		Model: DELL 1704FPV-
		Refresh Rate: 75