Creating the Filtering Channels

Now we have to prepare the image's three coloured filters. We are going to use the pointillist effect created by the Noise filter to simulate the potato starch grains.

1. Select Image > Mode > RGB Colours in order to restore the 24-bit coding.
   
   
2. Select the copy of the image (the colour version), choose Select > Select All, then Edit > Copy.
   
   
The colour image is copied into a layer (select Window > Show Layers, if it is not visible).


3. Reduce the window of the image copy. We no longer need it.
   
   
4. If layer 1 (that of the colour image) is not selected, click above. This displays the channels, as shown in Figure 2.
   
   


Figure 2: The Autochrome process also uses RGB channels.


5. Click on the Red channel.
   
   
6. Select Filter > Noise > Add Noise.
   
   
Maintain the default Uniform option so that the channel is evenly distributed.


7. Use the sliding scale to apply a quantity of noise within a range of 50 and 75 (the exact figure depends on the image).
   
   
The preview shows an effect that brings to mind the grain of a film. Click on OK when you are happy with the result.


The right filter. Try to avoid using the command Filter > Add Noise, as it applies the memorized values without giving you access to the filter settings dialog box.


8. Click on the Green channel and apply a noise value that is 5 points higher than that applied earlier on.
   
   
The distribution of the grains is now slightly different.


9. Click on the Blue channel and apply a slightly lower noise than that of the Red channel.
   
   
10. Click on the RGB channel to see how the "potato starch grains" have been distributed. If necessary, adjust the noise in the relevant channel if any of the main colours appear.