How to blend a gray and rgb image?

I need to blend a gray and rgb image, and my code is:

import vtk
import numpy as np
from vtk.util.numpy_support import vtk_to_numpy, numpy_to_vtk


def numpyToVTK(data, multi_component=False):
    if multi_component == False:
        if len(data.shape) == 2:
            data = data[:, :, np.newaxis]
        flat_data_array = data.transpose(2,1,0).flatten()
        vtk_data = numpy_to_vtk(num_array=flat_data_array, deep=True, array_type=vtk.VTK_FLOAT)
    else:
        assert len(data.shape)==3, 'only test for 2D RGB'
        flat_data_array = data.transpose(1, 0, 2)
        flat_data_array = np.reshape(flat_data_array, newshape=[-1, data.shape[2]])
        vtk_data = numpy_to_vtk(num_array=flat_data_array, deep=True, array_type=vtk.VTK_FLOAT)
    img = vtk.vtkImageData()
    img.GetPointData().SetScalars(vtk_data)
    img.SetDimensions(data.shape[0], data.shape[1], 1)
    return img

x = np.linspace(start=-1000, stop=500, num=256)
y = np.linspace(start=-1000, stop=500, num=256)
xx, yy = np.meshgrid(x, y)   # 为一维的矩阵
img = xx

rgb = np.zeros(shape=[256, 256, 3])
rgb[100:150, 100:150, 0] = 255
gray = numpyToVTK(img)
rgb = numpyToVTK(rgb, multi_component=True)

windowLevelColor = vtk.vtkImageMapToWindowLevelColors()
windowLevelColor.SetInputData(gray)
# windowLevelColor.SetWindow(400)
# windowLevelColor.SetLevel(0)
windowLevelColor.Update()

image_blender = vtk.vtkImageBlend()
image_blender.AddInputData(rgb)
image_blender.AddInputData(windowLevelColor.GetOutput())
image_blender.SetOpacity(0, 0.5)
image_blender.SetOpacity(1, 0.5)
image_blender.Update()

imageActor = vtk.vtkImageActor()
imageActor.GetMapper().SetInputData(image_blender.GetOutput())
# imageActor.GetMapper().SetInputData(gray)
# imageActor.GetMapper().SetInputData(windowLevelColor.GetOutput())
# imageActor.GetMapper().SetInputData(rgb)

ren = vtk.vtkRenderer()
ren.AddActor(imageActor)
ren.SetBackground(0.1, 0.2, 0.4)

renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(ren)
renWin.SetSize(400, 400)

iren = vtk.vtkRenderWindowInteractor()
iren.SetRenderWindow(renWin)

renWin.Render()

iren.Start()

I have tested that gray/rgb is OK:

# imageActor.GetMapper().SetInputData(gray)
# imageActor.GetMapper().SetInputData(windowLevelColor.GetOutput())
# imageActor.GetMapper().SetInputData(rgb)

The windlwLevelColor.GetOutput is:

The rgb is:

Unfortunately, the image_blender.GetOutput() is:

Is there anything wrong with my code? How can I blend the gray and rgb image? Any suggestion is appreciated!

The inputs for vtkImageBlend must have the same data type. Your RGB image is VTK_FLOAT, but the output of vtkImageMapToWindowLevelColors is VTK_UNSIGNED_CHAR.

If you change the type of your RGB image to VTK_UNSIGNED_CHAR, then the blending will work. I recommend that you add a data_type parameter to your numpyToVTK() function, so that you can do this:

rgb = np.zeros(shape=[256, 256, 3], dtype=np.uint8)
rgb[100:150, 100:150, 0] = 255
rgb = numpyToVTK(rgb, multi_component=True, data_type=vtk.VTK_UNSIGNED_CHAR)

Also, I recommend that you remove this line from your code, because it has no effect (as I will explain below):

image_blender.SetOpacity(0, 0.5)

For alpha blending, the blending coefficient for the base image always computed automatically. If you set the opacity of the second image to 0.5, then the blending coefficient for the base image is automatically set to 0.5. If you set the opacity of the second image to 0.25, the coefficient for the base image is automatically set to 0.75.

Also, I don’t know exactly what your requirements are, but it is common for the base image to be gray, and for the overlay image to be color. It is also fairly common for the overlay image to be RGBA, so that the alpha is part of the overlay image:

rgb = np.zeros(shape=[256, 256, 4], dtype=np.uint8) # RGBA
rgb[100:150, 100:150, 0] = 255 # set red=1.0
rgb[100:150, 100:150, 3] = 255 # set alpha=1.0

then apply the RGBA overlay to the gray image:

image_blender = vtk.vtkImageBlend()
image_blender.AddInputData(windowLevelColor.GetOutput())
image_blender.AddInputData(rgb)
# each pixel of the overlay will have an opacity of 1.0 * alpha
image_blender.SetOpacity(1, 1.0)
image_blender.Update()
1 Like

Thank you for your very kindly reply. It solved my problem.