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If the title was confusing to you, let me explain in further detail. I have Suzanne here, and I want her to show the surface reflection of an HDRI environment background picture I have set up, but in the actual render, I want to have a different color, or picture show in the background. But I still want her to reflect the first environment background. Is it possible?

Now yes, it is a posiblity to export a render with a transparent background, and edit said render in Photoshop or Afterfx if it's a video, but is it possible to do in blender?

transparent background

visible background

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If you want to use the HDRi to light the scene, but want a transparent background to be composited later with the background, or want to blur the HDR or maybe replace it all together, enable film transparent.

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Then you can use alpha over to combine the scene with any other image.

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If you enable the Environment output for the render layers, you can also make the HDRi background avalable to be used it as a different element in the compositor.

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The Env socket contains the information from the background so that it can be manipulated any way you want.

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Update. To do this in Eevee in 2.8

Just enable Film>Alpha>Transparent.

Then on the compositor use Alpha over to mix over the background you want to use.

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Alternatively you can use a background for the world that is only seen by the camera and not by other rays, therefore not contributing to the lights, reflectons,etc.

Set two different Environment background shaders and use the Is Camera Ray from a Light Path node to control the shader mix.

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You can achieve this with the compositer, render it with a transparent background.

Then mix the render output with the background image you want, with the factor being the alpha output of the render.

Composite Setup

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