Change color with object rotation

Question

I want to adjust the color of a material based on an object's rotation. I cannot do this with a driver, because the same material is used over a large number of objects: I would like each object to change color based on its own rotation value.

Is there a way to do this in either BI or Cycles? More generally, is there a way to change color based on an object property other than position?

Answer 1

Since blender 2.67 you can write a custom pynode or use Animation Nodes as shown below:

Note: Enable cycles first to access the color of the diffuse shader and use the expression node to get correct color values bellow 1.

Answer 2

Here is a hack:

• Read the color from object's vertex colors, this way your objects can share the same material with different colors:

  

• Change the vertex colors on frame change with python script. This won't have real-time performance but that shouldn't matter since you don't get material preview with vertex-colors in view-port anyway (it shows only in render).

  import bpy
  
  # this is how you add vertex-color map:
  obj.data.vertex_colors.new(name='Col')
  
  # this handler function, it will be called on every frame change
  def update_colors_from_rotation(scn):
      for obj in scn.objects:
          color = YOUR_FUNCTION(obj.rotation_euler)
          # color should be float array [R,G,B]
          # or you can access custom ID props here also: color = obj['base_color']
          # and do something with it
  
          # here is how you change the vertex-color map:
          for ipoly in range(len(obj.data.polygons)):
              for ivertex in obj.data.polygons[ipoly].loop_indices:
                  obj.data.vertex_colors['Col'].data[ivertex].color.r = color[0]
                  obj.data.vertex_colors['Col'].data[ivertex].color.g = color[1]
                  obj.data.vertex_colors['Col'].data[ivertex].color.b = color[2]
  
  # register the function in frame change handlers:
  bpy.app.handlers.frame_change_post.append(update_colors_from_rotation)
  

Answer 3

Here's another much simpler solution using the Vector Transform node.

The Combine XYZ represents a vector that indicates the 'front' of the object (in this case along the -Y axis - the face of Suzanne). This passes through a Vector Transform node set to Normal (so that the Location and Scale are ignored in the transform, leaving only the Rotation). Color Mix nodes then scale by 0.5 and add 0.5 to convert the vector (ranging from -1.0 to 1.0) into color space (0.0 to 1.0).

This produces the following result.

Note that this method isn't capturing the 'true' rotation Euler values but, instead, how one particular 'face' of the object is pointed. Rotating around the direction vector (in this case, rotating Suzanne around the Y axis) will result in no change in color but any other rotation will affect the resultant color.

Answer 4

Building on this answer, the Rotation details can be captured to an image - with each frame on a subsequent line of the image - and interfaced into the Cycles material using an Image Texture node.

The following script will run through each frame of the scene and create an image (width = number of objects, height = number of frames) containing the Rotation details encoded stored in each pixel.

import bpy
import os

width = len(bpy.data.objects)
height = bpy.context.scene.frame_end

# Create the image
image = bpy.data.images.new("object_rotations_"+str(width)+"x"+str(height), width=width, height=height,float_buffer=True)

# Create an empty array of pixel data (each will hold R, G, B, A values as floats)
pixels = [None] * width * height

# For each frame
for frame in range(0,height):
    bpy.context.scene.frame_set(frame)

    # For each object
    for x in range(0,width):
        y = frame
        object = bpy.data.objects[x]
        pixels[(y*width)+x] = [object.rotation_euler[0], object.rotation_euler[1], object.rotation_euler[2],1.0]
        object.pass_index = x

# 'flatten' the array
pixels = [chan for px in pixels for chan in px]

# Store the pixels in the image
image.pixels = pixels
image.update()

# Save image to temporary file and pack it (from answer by @sambler)
scn = bpy.data.scenes.new('img_settings')
scn.render.image_settings.file_format = 'OPEN_EXR'
scn.render.image_settings.color_mode = 'RGBA'
scn.render.image_settings.color_depth = '32'
img_path = bpy.path.abspath('//')
img_file = image.name+'.exr'
image.save_render(img_path+img_file, scene=scn)
try:
    # Remove the scene (pre-2.78)
    bpy.data.scenes.remove(scn)
except:
    # Remove the scene, (Blender 2.78+)
    bpy.data.scenes.remove(scn, do_unlink=True)


bpy.data.images.remove(image)
bpy.ops.image.open(filepath=img_path+img_file)
image = bpy.data.images[img_file]
image.pack()

os.remove(img_path+img_file)

# Add a fake user to ensure the image is retained
image.use_fake_user = True

This will capture the rotation details of each object into a row of pixels in a generated image with one row for each frame in the timeline. Each pixel's RGB values represent the XYZ Euler rotation values for the corresponding object on that particular frame.

The material can extract the rotation of each object using some relatively simple maths - set the Image Texture to point to the generated image (and set to Non-color Data).

Set keyframes on the highlighted Frame value node - to vary the frame in step with the actual timeline frame (and set Interpolation to Linear) - this is the easiest method of getting the frame number into the Cycles material. Also, ensure the highlighted Width and Height Maths Divide nodes match the Width and Height of the generated image.

This can produce the following effect, varying the color based on the object rotation.

Note that if the rotation of the objects (or the number of objects) is changed then the script will need to be re-run to re-capture the object rotations to the image.