If you check the docs, you can see under NodeSocket(bpy_struct):

type Data type Is

Type: enum in [‘CUSTOM’, ‘VALUE’, ‘INT’, ‘BOOLEAN’, ‘VECTOR’, ‘STRING’, ‘RGBA’, ‘SHADER’], default ‘VALUE’

What happens, if you connect sockets of differing types? In particular, RGBA connected to SHADER (Mix -> Material Output), and RGBA connected to VALUE (Image Texture -> ColorRamp)? Can someone tell how blender converts between the differing types?

  • $\begingroup$ What are you asking about? These are simply types of sockets (the little dots on the sides of nodes), where usually you'll make connections between the same types (color > color, value > value...). $\endgroup$
    – Greg Zaal
    May 24 '15 at 14:16
  • $\begingroup$ I'm asking because I find it weird, that I can connect RGBA socket to SHADER socket. Why? They are of different type. $\endgroup$ May 24 '15 at 14:34
  • 1
    $\begingroup$ blender assumes that you know enough to follow the color code at least $\endgroup$
    – Chebhou
    May 24 '15 at 16:14
  • $\begingroup$ You can connect any socket type to any other socket type and usually the conversion is done automatically - the exception is the SHADER type, where only a shader socket plugged into another shader socket will work (although there's nothing physically stopping you from trying) $\endgroup$
    – Greg Zaal
    May 24 '15 at 18:05
  • $\begingroup$ @GregZaal Can you enumerate how the conversions take place, i.e. how RGBA is converted to VALUE, for example. There are many possibilities, such as a weighted sum, taking only one of 4 values, etc... I'm also interested where in blender's code I could look up these conversions taking place. Also, you forget that I have RGBA plugged into the SHADER socket. It works, try it, though how? $\endgroup$ May 24 '15 at 18:29

RGB(A) to Value

It is neither a weighted sum nor the max channel (as is the case for Value in HSV) - instead it's something I don't understand at all, something to do with the black magic of color management, but I'm happy to write it here anyway :)

In node_shader_valToTgb.c is the function node_shader_exec_rgbtobw, which calls IMB_colormanagement_get_luminance(col). Inside that function, we have dot_v3v3(luma_coefficients, rgb) - the dot product of the input color and some apparently random numbers at the top of that file: luma_coefficients[3] = { 0.2126f, 0.7152f, 0.0722f };

Recreating that as a node setup in blender gives the same result as you'd get from plugging the color into a Color Ramp node or RGB to BW node:


The cube render is black meaning the difference = 0

Shader nodes are always only RGB and never include an alpha channel. However, in the compositor they do. When converting RGBA to a Value in the compositor, the alpha channel is simply ignored - only the RGB values are used.

RGB(A) to Vector

The alpha channel is simply discarded - Red becomes X, Green becomes Y, Blue becomes Z.

We can prove this by plugging a render into a Vector Curves node, and plugging that into a Viewer node - notice that the background becomes black (as the RGB values in that space were all 0):


Vector to RGB(A)

It's simply the opposite of what's said above. X becomes Red, Y becomes Green, Z becomes Blue. The alpha is always assumed to be 1.0.

Anything to Shader

This is the one exception - nothing is ever automatically converted to a shader, the result is always simply black, no matter what color or value you plug into it.

Personally I think this design is silly - if you plug a color straight into the material output (without going through any shaders first), you're expecting the result to be shadeless. But alas, this is not the case.

Value to RGB(A)/Vector

This is pretty obvious I think, but just for completeness: Whatever the value is becomes the same for each channel/axis when converted to RGBA/Vector (though the alpha channel simply = 1)

Have I missed anything?

  • 2
    $\begingroup$ About those "random" numbers, see Relative luminance and Luminosity function. $\endgroup$
    – user7952
    May 24 '15 at 21:35
  • $\begingroup$ The RGBA->Value conversion IS a weighted sum. I am positive, that conversion to SHADER are possible, after all the socket Material Output is of type SHADER and you can connect a RGBA socket to it. $\endgroup$ May 25 '15 at 11:09
  • $\begingroup$ Are you saying you can connect an RGB socket to a shader socket and it renders something other than black? Need a screenshot to believe that... $\endgroup$
    – Greg Zaal
    May 25 '15 at 14:32

node_socket_copy_default_value() seems to be responsible for moving data between node sockets, but it doesn't go along with the real behavior so I may be wrong, and here is some links further invistigation :

node related definitions
node directory in source code


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.