On my model I have red areas on the grey texture. I accomplish this by adding a 2nd texture and using an RGB curve to single out the dark red only.

I would like to get the red areas as you can see below, to glow as if they where hot.

enter image description here

Current nodes

enter image description here

Using Emission in place of the Diffuse BSDF for the 2nd texture: enter image description here

I've tried adding emission in there, but it seems to just increase the amount of red over the surface of the rock, and cranking it's strength up increased the amount of red though the brightness stays about the same. I want to keep the amount of red the same, but make it seem as if it where hot and giving off light.

Edit: There is a pretty large chance that I'm going about this completely wrong though.

  • $\begingroup$ You have 3 textures. 2 have UV Modified. Was that intentional? Have you used Viewer Nodes to verify that steps meet your expectation? I am not sure why you are adding a 2D texture when the rock has plenty of color variation. Can you explain? Can you just use one texture as input multiple times? $\endgroup$ May 17 '15 at 22:14
  • $\begingroup$ You will have to excuse my ignorance atomic, I am fairly new to blender. So I may not understand the full consequence of having the 2nd texture UV modified. There does not seem to be viewer nodes on materials (under Output), I can find and use them in the compositor though. $\endgroup$ May 17 '15 at 22:20
  • $\begingroup$ I cannot see you original red texture. Also if you only have one image then things are simpler. If you have a image sequence I do not understand how the two images combine in a sensible way. $\endgroup$ May 17 '15 at 23:17
  • $\begingroup$ You last node image is the most valuable. The older ones are less valuable. $\endgroup$ May 17 '15 at 23:35
  • The weight factor of the red component is used for mix shader. The weight of the red component my be unsuitable. It may be too small. If the red image has only red components keep this in mind for the math. If red highlight image were actually bright white and black with maximum white values in some areas that would be desirable. Multiply the weight of the red by 3 with a math node. When red is normalized RGB->BW it may have been divided by three. What you may have intended was ... to use the red color only as a basis for brightness and do not consider green and blue. A single red color can only have 1/3 the brightness of bright white. Some mathematical liberties taken. It would be best if your red highlight image has very bright red areas. Scaling by a value greater than 1, ... 3 or 4 for example, will help to bring the brightness in the range you desire. The highlight image may exist as black and white, rather than red, and be recolored to any other color in the Compositor.

  • Assuming you are using materials with emission shader. Increase the strength of emission for blinding brightness.

  • Your color curve may be unsuitable. All brightness may have been wiped out. Remedy. Make everything too bright. Bring bright side to the middle. Then reduce artistically. My compositor color ramp may serve as an example. Notice how the white color dominates most of the color ramp.

  • Know that the compositor can user Alpha Over node to lay images with transparency on top of one another.

  • Use an EMISSION shader and diffuse shader to the mix shader, if you continue to use the materials approach. You clearly have a better chance of getting a bright color with the emission shader.

  • Compositor, compositing pixels, compositing nodes, with a color ramp can threshold colors and produce bright colors. Next blur the edges. Materials are a different approach.

  • Below is a compositor image of your original images. I have used a color ramp to threshold the brightest color and dilated them to white. White is easier to see then red. The border has not been removed. This is an illustrative technique example not artistic for a single picture.

enter image description here

  • Make sure you texture coordinates are correct for UV
  • $\begingroup$ Thank you for the reply atomic. I have tried using an emission shader in place of the diffuse shader to go to the mix shader. Here are the results: i.imgur.com/WsjxtGt.png turning it's strength up results in more of the object turning red, but not necessarily any of the red becoming brighter. $\endgroup$ May 17 '15 at 22:22
  • $\begingroup$ Thank you for the more detailed answer atomic. I ended up figuring was going about this in a fundamentally incorrect way. I just needed a texture for emission only, so I went ahead and did that. I posted an answer with the process I took. $\endgroup$ May 18 '15 at 0:00

So it turns out I was going about this in a fundamentally incorrect way. What I ended up doing was taking the base texture, reducing it's color balance in GIMP to the minimum, then using GIMP's color curve darkened it up to the point that the majority of the remaining colors where black.

I then imported that into blender and used that texture for the emission. As detailed below:

Base Texture:enter image description here

Reduced Color Levels:enter image description here

Adjust Color Curve: enter image description here

Final Product: enter image description here

  • $\begingroup$ I still have suspicion that my first bullet point applies even to the picture above. Consider scaling the mixing factor by 3. The wonderful GIMP is a convenience but not necessary. Only you can judge the artistry. $\endgroup$ May 18 '15 at 0:04
  • $\begingroup$ Ah yes, I had not reloaded the page and missed the extension of your first bullet point. I went ahead and did that and the result is definitely much brighter. Thank you for the understandable, and detailed explanation. However, how do I maintain the quantity of colors that where expressed in the texture through the compositor? $\endgroup$ May 18 '15 at 0:10
  • $\begingroup$ I may not understand your question. The compositor can change pixels just as GIMP can. You can combine a rendered 3D image with external textures loaded from secondary storage (the hard disk). Is the rock a 3D model or image mapped to plane. Are you manipulating a single image or image sequence, or video? If the rock is 3D model how do you coordinate that unrelated texture? Blender has internal textures such as Voronoi with parameters which can employ keyframes. Consider searching for Blender Compositor Tutorial on a video site of our choice. This site is not really about tutorials. $\endgroup$ May 18 '15 at 7:59

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