(In reply to stephen from comment #0)
> Created attachment 131991 [details]
> Inconsistent line tracing with line tool
> 
> SUMMARY
> I'm using a Krita plus build version 4.4.1 alpha(git b20cb06)
> There's a bug with the line tool.
> 
> STEPS TO REPRODUCE
> 1. Select the line tool either by holding V button or by clicking on the
> tool icon.
> 2. Start painting, trace, then release.
> 3. Repeat step 2 multiples times to see what happens.
> 
> OBSERVED RESULT
> Tracing lines is inconsistent. There are typical failures : either only part
> of the distance is traced, or nothing happens, even at full pen pressure.
> 
> EXPECTED RESULT
> Consistent and correct lines traced according to pen pressure and distance
> between the starting point and the final point.
> 
> OS Version : Windows 10 1909
> 
> ADDITIONAL INFORMATION
> Krita
> 
>  Version: 4.4.1-alpha (git b20cb06)
>  Languages: en_US, en, en_US, en, en_US, en, en_US, en, en_US, en, en_US,
> en, en_US, en, en_US, en, en_US, en
>  Hidpi: false
> 
> Qt
> 
>   Version (compiled): 5.12.9
>   Version (loaded): 5.12.9
> 
> OS Information
> 
>   Build ABI: x86_64-little_endian-llp64
>   Build CPU: x86_64
>   CPU: x86_64
>   Kernel Type: winnt
>   Kernel Version: 10.0.18363
>   Pretty Productname: Windows 10 (10.0)
>   Product Type: windows
>   Product Version: 10
> 
> OpenGL Info
>  
>   Vendor:  "Google Inc." 
>   Renderer:  "ANGLE (NVIDIA GeForce GT 640M Direct3D11 vs_5_0 ps_5_0)" 
>   Version:  "OpenGL ES 3.0 (ANGLE 2.1.0.57ea533f79a7)" 
>   Shading language:  "OpenGL ES GLSL ES 3.00 (ANGLE 2.1.0.57ea533f79a7)" 
>   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 3.0, options
> QFlags<QSurfaceFormat::FormatOption>(), depthBufferSize 24, redBufferSize 8,
> greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8,
> samples 0, swapBehavior QSurfaceFormat::DefaultSwapBehavior, swapInterval 0,
> colorSpace QSurfaceFormat::DefaultColorSpace, profile 
> QSurfaceFormat::NoProfile) 
>      Version: 3.0
>      Supports deprecated functions false 
>      is OpenGL ES: true 
> 
> QPA OpenGL Detection Info 
>   supportsDesktopGL: true 
>   supportsAngleD3D11: true 
>   isQtPreferAngle: true 
> 
> Hardware Information
> 
>   GPU Acceleration: angle
>   Memory: 12126 Mb
>   Number of Cores: 4
>   Swap Location: C:/Users/Yed/AppData/Local/Temp
> 
> Current Settings
> 
>   Current Swap Location: C:/Users/Yed/AppData/Local/Temp
>   Current Swap Location writable: true
>   Undo Enabled: true
>   Undo Stack Limit: 400
>   Use OpenGL: true
>   Use OpenGL Texture Buffer: true
>   Use AMD Vectorization Workaround: false
>   Canvas State: OPENGL_SUCCESS
>   Autosave Interval: 300
>   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: 1
> 	Screen: 0
> 		Name: \\.\DISPLAY1
> 		Depth: 32
> 		Scale: 1
> 		Resolution in pixels: 1600x900
> 		Manufacturer: 
> 		Model: 
> 		Refresh Rate: 60
> 
> Current Settings
> 
>   Current Swap Location: C:/Users/Yed/AppData/Local/Temp
>   Current Swap Location writable: true
>   Undo Enabled: true
>   Undo Stack Limit: 400
>   Use OpenGL: true
>   Use OpenGL Texture Buffer: true
>   Use AMD Vectorization Workaround: false
>   Canvas State: OPENGL_SUCCESS
>   Autosave Interval: 300
>   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

It seems the issue happens when airbrush option is on in the brush settings.

For the 4.2.9 appimage onwards, the line tool does not paint at all when the Airbrush option is enabled in a Pixel Engine brush preset.
This is the case for paint or vector layers, and for mouse or stylus.

Krita

 Version: 4.4.1-alpha (git e02f58a)
 Languages: en_GB, en, en, en_GB, en
 Hidpi: false

Qt

  Version (compiled): 5.12.9
  Version (loaded): 5.12.9

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-10-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 440.100" 
  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: 1860
  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

Hi stephen, I tested the line tool and at least on macOS results are as expected.

Could you record a video of what you are getting? Line tool varies depending on the pressure during the entire line stroke gesture. So you can trace different pressure levels depending on the distance. If you see no line at the very begging or end it might mean you almost had no pressure on your stroke and the preset you are using does not paint at that "pressure" at that point in the line. (its quoted as the line can vary depending on any sensor you set).

Readjust your brush preset so it paints at any sensor state during the tracing.

Dear Bug Submitter,

This bug has been in NEEDSINFO status with no change for at least
15 days. Please provide the requested information as soon as
possible and set the bug status as REPORTED. Due to regular bug
tracker maintenance, if the bug is still in NEEDSINFO status with
no change in 30 days the bug will be closed as RESOLVED > WORKSFORME
due to lack of needed information.

For more information about our bug triaging procedures please read the
wiki located here:
https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging

If you have already provided the requested information, please
mark the bug as REPORTED so that the KDE team knows that the bug is
ready to be confirmed.

Thank you for helping us make KDE software even better for everyone!

(In reply to vanyossi from comment #3)
> Hi stephen, I tested the line tool and at least on macOS results are as
> expected.
> 
> Could you record a video of what you are getting? Line tool varies depending
> on the pressure during the entire line stroke gesture. So you can trace
> different pressure levels depending on the distance. If you see no line at
> the very begging or end it might mean you almost had no pressure on your
> stroke and the preset you are using does not paint at that "pressure" at
> that point in the line. (its quoted as the line can vary depending on any
> sensor you set).
> 
> Readjust your brush preset so it paints at any sensor state during the
> tracing.

My first comment contains already a video recording of my tests.
You may check that to see it helps.

Ahab already confirmed and I'm confirming again.

To see the behaviour, it's important to check "Airbrush" setting in the Brush Editor.

1. Select "Basic-2 Opacity" brush.
2. Go to Brush Editor.
3. Uncheck "Pen Settings" in "Opacity". (See making lines: it all works correctly).
4. Check "Airbrush".

OBSERVED
- Sometimes no lines
- Sometimes parts of lines
- Note that preview works (after a moment) and that unchecking "Use sensors" in Tool Options doesn't do anything.

EXPECTED
Lines made in a very similar way to when Airbrush was unchecked.

*** This bug has been marked as a duplicate of bug 447230 ***