Mixbox: Practical Pigment Mixing for Digital Painting

Mixbox is a pigment mixing black-box. You pass RGB colors in and get the mixed RGB out. Internally, Mixbox treats the colors as if they were made of actual real-world pigments. It uses the Kubelka & Munk theory to predict the color of the resulting mixture. This way, Mixbox achieves that blue and yellow mix to green, the same way real pigments do.

• Paper: https://scrtwpns.com/mixbox.pdf
  
• Video: https://youtu.be/ATzVPVNp1qA
  
• Talk: https://youtu.be/_qa5iWdfNKg
  
• Demo: https://scrtwpns.com/mixbox/painter
  

Mixbox is shipping in Rebelle 5 Pro as the Rebelle Pigments feature.

Usage

The simplest way to use Mixbox is with the lerp interface:

#include <stdio.h>
#include "mixbox.h"

int main()
{
  unsigned char r1=252, g1=211, b1=0;  // bright yellow
  unsigned char r2=0,   g2=0,   b2=96; // deep blue
  float t = 0.5;
  unsigned char r,g,b;
  
  mixbox_lerp_srgb8(r1,g1,b1,   // first color
                    r2,g2,b2,   // second color
                    t,          // mixing ratio
                    &r,&g,&b);  // result

  printf("%d %d %d\n",r,g,b);
}

Alternatively, one can use the latent interface. This allows mixing multiple RGB colors at once using arbitrary weights:

float latent1[MIXBOX_NUMLATENTS];
float latent2[MIXBOX_NUMLATENTS];
float latentMix[MIXBOX_NUMLATENTS];

mixbox_srgb8_to_latent(r1,g1,b1,latent1);
mixbox_srgb8_to_latent(r2,g2,b2,latent2);

for(int i=0;i<MIXBOX_NUMLATENTS;i++)
{ 
  latentMix[i] = (1.0f-t)*latent1[i] + t*latent2[i]; 
}

mixbox_latent_to_srgb8(latentMix,&r,&g,&b);  

Demo

This is a toy painting app with soft round brush and a smudge tool. It runs two color mixing implementations in parallel: one based on Mixbox and the other that performs ordinary RGB mixing. The app allows switching between them on the fly, showing the differences between pigment-based mixing and the normal RGB mixing. To launch the demo in your browser, please click here.

License

Copyright (c) 2022, Secret Weapons. All rights reserved.
This code is for non-commercial use only. It is provided for research and evaluation purposes.
If you wish to obtain commercial license, please contact: [email protected]