# vtkCompositePolyDataMapper keeps per-block color after switching to scalar coloring (9.7.x, Windows)
**URL:** https://discourse.vtk.org/t/vtkcompositepolydatamapper-keeps-per-block-color-after-switching-to-scalar-coloring-9-7-x-windows/16550
**Category:** Support
**Created:** [October 6, 2026, 12:32pm UTC](https://discourse.vtk.org/t/vtkcompositepolydatamapper-keeps-per-block-color-after-switching-to-scalar-coloring-9-7-x-windows/16550 "2026-10-06T12:32:52Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@MaxNumerique](https://discourse.vtk.org/u/MaxNumerique)
#### Post date: [October 6, 2026, 12:32pm UTC](https://discourse.vtk.org/t/vtkcompositepolydatamapper-keeps-per-block-color-after-switching-to-scalar-coloring-9-7-x-windows/16550/1 "2026-10-06T12:32:52Z")
Hi,
After upgrading to VTK 9.7.1, we’re seeing a rendering issue with **vtkCompositePolyDataMapper** and **vtkCompositeDataDisplayAttributes**.
We first give a set of blocks a solid color ( **SetBlockColor** ) and render. We then switch the same blocks to scalar coloring: **RemoveBlockColor** , per-block LUT, array name, scalar range, scalar mode and **SetBlockScalarVisibility** (true). After rendering again, some or all of the blocks still show the solid color.
It happens only on Windows (GitHub Actions windows-2022, offscreen), and only intermittently.We have never seen it on Linux, and never with 9.6.2, which we are pinning for now. Our guess is that it’s related to the new mapper caching in 9.7.0 (TBO/IBO cache, partial updates).
The attached script reproduces the sequence of calls and counts the pixels still drawn with the solid color:
```auto
"""Blocks keep their solid color after switching to scalar coloring (VTK 9.7.x).
Usage: python repro_block_color_to_scalars.py [iterations]
"""
import sys
import vtk
from vtkmodules.util.numpy_support import vtk_to_numpy
MAGENTA = (1.0, 0.0, 1.0)
def run_once() -> int:
blocks = vtk.vtkMultiBlockDataSet()
for i in range(40):
plane = vtk.vtkPlaneSource()
plane.SetOrigin(i % 8, i // 8, 0)
plane.SetPoint1(i % 8 + 1, i // 8, 0)
plane.SetPoint2(i % 8, i // 8 + 1, 0)
plane.SetResolution(10, 10)
plane.Update()
polydata = plane.GetOutput()
scalars = vtk.vtkFloatArray()
scalars.SetName("values")
for p in range(polydata.GetNumberOfPoints()):
x, y, _ = polydata.GetPoint(p)
scalars.InsertNextValue((x * 37 + y * 11) % 200 - 100)
polydata.GetPointData().AddArray(scalars)
blocks.SetBlock(i, polydata)
mapper = vtk.vtkCompositePolyDataMapper()
attributes = vtk.vtkCompositeDataDisplayAttributes()
mapper.SetCompositeDataDisplayAttributes(attributes)
mapper.SetInputDataObject(blocks)
actor = vtk.vtkActor()
actor.SetMapper(mapper)
renderer = vtk.vtkRenderer()
renderer.AddActor(actor)
window = vtk.vtkRenderWindow()
window.SetOffScreenRendering(True)
window.SetSize(600, 400)
window.AddRenderer(renderer)
renderer.ResetCamera()
block_list = [blocks.GetBlock(i) for i in range(40)]
# 1. Solid color on every block
for block in block_list:
attributes.SetBlockColor(block, MAGENTA)
window.Render()
# 2. Switch every block to scalar coloring
lut = vtk.vtkColorTransferFunction()
lut.AddRGBPoint(-100, 0.1, 0.1, 0.4)
lut.AddRGBPoint(100, 0.9, 0.9, 0.9)
for block in block_list:
block.GetPointData().SetActiveScalars("values")
attributes.RemoveBlockColor(block)
attributes.SetBlockLookupTable(block, lut)
attributes.SetBlockArrayName(block, "values")
attributes.SetBlockScalarRange(block, vtk.vtkVector2d(-100, 100))
attributes.SetBlockScalarMode(block, vtk.VTK_SCALAR_MODE_USE_POINT_DATA)
attributes.SetBlockScalarVisibility(block, True)
mapper.ScalarVisibilityOn()
mapper.SetColorModeToMapScalars()
mapper.Modified()
window.Render()
# 3. Count pixels still drawn with the solid color (expected: 0)
grabber = vtk.vtkWindowToImageFilter()
grabber.SetInput(window)
grabber.Update()
rgb = vtk_to_numpy(grabber.GetOutput().GetPointData().GetScalars())
window.Finalize()
return int(((rgb[:, 0] > 240) & (rgb[:, 1] < 15) & (rgb[:, 2] > 240)).sum())
if __name__ == " __main__":
iterations = int(sys.argv[1]) if len(sys.argv) > 1 else 20
print("VTK", vtk.vtkVersion.GetVTKVersionFull())
failures = sum(run_once() > 0 for _ in range(iterations))
print(f"{failures}/{iterations} failing iterations")
sys.exit(1 if failures else 0)
```
---
### Author:  [@jaswantp](https://discourse.vtk.org/u/jaswantp)
#### Post date: [October 6, 2026, 12:42pm UTC](https://discourse.vtk.org/t/vtkcompositepolydatamapper-keeps-per-block-color-after-switching-to-scalar-coloring-9-7-x-windows/16550/2 "2026-10-06T12:42:50Z")
Hello @MaxNumerique,
Thank you for sharing the reproducer.
> [@MaxNumerique](#):
>
> Our guess is that it’s related to the new mapper caching in 9.7.0 (TBO/IBO cache, partial updates).
If you are referring to [vtk/vtk!13324](https://gitlab.kitware.com/vtk/vtk/-/merge_requests/13324), this is likely not the cause. The TBO cache is enabled only for the **low-memory mapper** , which is the default in WebAssembly but **not** on desktop.
Since your script uses `vtkCompositePolyDataMapper` (which utilizes `vtkOpenGLBatchedPolyDataMapper` for leaves) rather than explicitly creating a `vtkOpenGLLowMemoryBatchedPolyDataMapper`, the issue likely stems from a different source.
Feel free to open an issue - [https://gitlab.kitware.com/vtk/vtk/-/work\_items/new](https://gitlab.kitware.com/vtk/vtk/-/work_items/new)
---
### Author:  [@MaxNumerique](https://discourse.vtk.org/u/MaxNumerique)
#### Post date: [October 6, 2026, 12:57pm UTC](https://discourse.vtk.org/t/vtkcompositepolydatamapper-keeps-per-block-color-after-switching-to-scalar-coloring-9-7-x-windows/16550/3 "2026-10-06T12:57:34Z")
Thank you !
Just opened an issue [here](https://gitlab.kitware.com/vtk/vtk/-/work_items/20188)