How can I combine two colored textures so that the final image is blended to match a specific lighting setup

Question

I have been working on using Blender Cycles to bake textures but it renders very slowly so I have to render at low resolutions. This is fine for the indirect diffuse lighting but really makes a mess of high resolution/high frequency textures.

To deal with this, I would like to only use Cycles to bake a low-res diffuse texture and then scale it up to match the high-res base texture and combine them using GIMP.

Unfortunately, none of the simple default layer combining methods work very well for this. Darken, and Multiply work well enough for the brightness but ruin the color.

I assume that I will have to use several layers in combination with different settings to achieve this but I'm not sure where to start.

Here are some textures. I'd like to combine the first two textures in a way that achieves the results shown in images #3 and #4.

(i) a basic colored procedural texture baked with no lighting(Blender Internal).

(ii) a texture baked with Cycles diffuse lighting only

(iii) a texture baked with both of the above(this is what I'm hoping to avoid), I would like to use GIMP to get this result using textures (i) and (ii).

(iv) The final scene with lighting baked in using the textures that were baked the same as image #3.

Answer 1

This is not possible to do accurately with GIMP because GIMP does not support HDR (High Dynamic Range) images. You see, when you load a image in GIMP it is reduced to 8 bit per color channel, but Blender works with 32 bit per color channel. GIMP can't process colors that are darker than black or whiter than white, but Blender can.

You'll need a application that can process that data too, for example Krita or Photoshop.

• Export the baked light texture from Blender as OpenEXR, this makes sure that no information is lost.
• Load both textures into one document.
• Set the light layer to multiplicative.
• Save it as PNG or whatever format you need.

Answer 2

I am answering on behalf of Leon Cheung who answered a similar thread which uses the Blender Compositor to do this with nodes. Here is his response.

https://blender.stackexchange.com/questions/7528/how-can-i-combine-two-colored-textures-so-that-the-final-image-is-blended-to-mat

He included a sample Blender file that does this.

It looks like Multiply is different for both software packages.

Multiplying with GIMP leaves the colors looking flat and darker than they should be. I tested this with an older PhotoShop and the problem is there as well.

Using the compositor, the final image came out looking just as expected.

Maybe GIMP is failing to normalize the values, or it's normalizing something that it shouldn't, who knows.

Answer 3

A good way to make it better is using the compositor. The image had colors in tact a bit more. I had a similar issue a bit ago on something else.