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I'm still confused with the difference between metallic and specular , and i could not found anything on web search that can clear my confusion between the two . Based on my knowledge , metallic is less reflective than specular , but what else ?

I've read https://docs.blender.org/manual/en/latest/render/shader_nodes/shader/principled.html but the way it is explained is kind of high level for beginner like me . I would be very thank you if anyone can explain the difference in a way that is easier to understand .

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There's plenty of room for confusion here. Metallic materials have specular, so in that sense, it's inappropriate to draw a distinction between specular and metallic-- the more proper distinctions would be between specular and diffuse and between metallic and dielectric. In this sense, trying to distinguish between metallic and specular is a little like asking, what is the difference between oranges and freedom?

However, metallic and specular are used to describe two different types of shaders that are sometimes used in conjunction. They are often referred to as the metallic workflow vs. the specular workflow.

A metallic workflow does not mean that it only represents metals. Anything with a "metalness" input, like the Principled BSDF, represents a metallic workflow, and 0.0 is a perfectly valid input for metalness.

The metalness workflow makes certain assumptions that are generally valid, physically speaking. It assumes that metals have no diffuse component-- they have a black diffuse. It assumes that metals can have colored specular, while non-metals cannot. (You can find some variation here, as with principled's specular tint, but note that you can't change the hue of the specular from the base color, only the saturation.) In the case of Blender's Principled BSDF, it also builds in a particular Fresnel response.

In contrast, a specular workflow is based less on physical features, and more on how 3D computer graphics work-- and, maybe, more on how artists think. What makes something a specular workflow is that it has a separate diffuse and specular component. Because of this, the diffuse and specular components can be completely different hues. It is less likely to be energy conserving, and is more likely to just compute a diffuse, compute a specular, and add them together, meaning that the albedo of a specular workflow material can be hard to calculate-- it's not just a function of the diffuse and specular colors, but of the roughness and Fresnel as well.

Neither of these workflows represents a single material or shader. They represent classes of shaders. Every decent game dev is going to start with one or both of these and add their own details as they need, in order to show the pictures that they want to show. Some Blender artists are hard-core PBR uber alles, but many others tune their materials to render the picture they see in their heads, which isn't always physically based.

Metalness workflow is, usually, more physically realistic than specular. But the assumptions it makes are not always valid. Real objects are not made of surfaces, but of volumes, just as real objects are not made out of macroscopic flat triangles. There is no such thing as a perfect metal that a metalness workflow simulates. There exists no shortage of real materials that are not properly represented by either a metalness or a specular workflow. Some exotic materials-- mother of pearl, for example-- are much better simulated with a specular workflow than with a metallic workflow.

In the case of Blender, the Principled BSDF is a metalness workflow, and the Specular shader (available only in Eevee) is a specular workflow. You can use both if you want-- you can mix shader between them if you want.

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    $\begingroup$ Great answer, thanks for the insight :). Still, what IS the difference between oranges and freedom...? $\endgroup$ Jul 7 '21 at 6:17
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Per Blender documentation, Metallic is just a value that represents how Metallic a material is. Usually the value is either 0 (non metal) or 1 (metal). Think of Specular as simply the strength of the highlight for the material. (0 = no highlight, 1 = normal highlight.)

In reality, specular is a certain type of reflection for a material. When you shine a bright light at something, that something then reflects that light. The center of that reflected light you see is the specular reflection, while the surrounding of that light is the diffuse reflection.

enter image description here

source: https://docs.blender.org/manual/en/latest/render/shader_nodes/shader/principled.html

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  • $\begingroup$ And how do i define how Metallic is an object and how Specular is an object ? I know they are about reflection but how do i differentiate between the two ? $\endgroup$ Jul 7 '21 at 2:29
  • $\begingroup$ an object is either metallic or it is not, so it's easy to choose (exactly like in reality). The Specular value depends on your object, it determines the intensity of the reflection, so choose the value that visually suits the best. If your object is metallic, specular doesn't seem to change a lot. $\endgroup$
    – moonboots
    Jul 7 '21 at 5:27
  • $\begingroup$ @user6668201 Specular is the amount of dielectric (non-metallic) specular reflection. So if you are using "real" settings like either metallic (1) or dielectric (0) and nothing inbetween, then the Specular value has absolutely no influence for a metallic material and only affects a non-metallic reflection. The specular reflection on a metallic material is mostly seen on the edges facing away from the view, the fresnel settings play a role here. $\endgroup$ Jul 7 '21 at 6:47
  • $\begingroup$ ...that said, unless the metallic surface has no clearcoat, the fresnel effect should have no influence on the reflection, so if you are creating a metallic material it would be best to set the specular to 0. $\endgroup$ Jul 7 '21 at 7:39
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From a visual point of view, a metallic surface colors the reflection with its color: all reflections in a blue metallic surface will be tinted blue. In contrast, a specular surface will have color-neutral reflections regardless of the base color. In other words, reflections on a specular object are added to the base color, while reflections on a metallic object are multiplied by it. This is an oversimplification, but helpful for visualization of the differences.

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