Add more information about ink modes

drawing.md

@@ -4,10 +4,11 @@ Using tools like the Pencil ![Pencil tool icon](tools/pencil-tool.png) (`B` key)
 you can use `Left click` to paint with the [Foreground color](color-bar.md), or `Right click`
 to paint with the Background color.
 
-(*Work-in-progress*)
+The tool behavior is modified by the [active ink](ink.md).
 
 ---
 
 **SEE ALSO**
 
-[Shading](shading.md)
+[Ink](ink.md) |
+[Tool Bar](tool-bar.md)

ink.md

@@ -0,0 +1,55 @@
+# Ink
+
+![Inks](ink/inks.png)
+
+The ink modifies the way the [active tool](tool-bar.md) paints. The
+default ink is the *Simple Ink*.
+
+## Simple Ink
+
+It acts in the following way:
+
+1. If the [foreground color](color-bar.md#foreground-color) is opaque
+   (alpha = 255 = 100%), it paints with the given opaque color.
+1. If the color has alpha (0 < alpha < 255), it composite the color
+   with the layer surface.
+1. If the color is transparent (alpha = 0, Mask color), the tool acts
+   like an Eraser.
+
+## Alpha Compositing
+
+It merges the [foreground color](color-bar.md#foreground-color) with
+the layer surface depending on the alpha value of the foreground color:
+
+1. If alpha = 255 = 100%, the foreground color will be completely opaque.
+1. If alpha = 128 = 50%, the foreground color is merged 50% with the layer surface color.
+1. If alpha = 0 = 0%, the painting has no effect because the color is completely transparent.
+
+## Copy Alpha+Color
+
+It replaces the layer surface pixels with the active foreground color
+with its alpha value. It doesn't make any kind of alpha compositing,
+it just takes the active color and put it exactly as it is in the
+destination pixel.
+
+E.g. In this case, if alpha = 128 = 50%, the final color will be the
+same as the foreground color with alpha = 128, ignoring the layer
+surface.
+
+## Lock Alpha
+
+In this case the original alpha values from the layer surface are
+kept, and only the RGB color components are replaced from the
+foreground color.
+
+## Shading
+
+See the [shading section](shading.md) for more information about this
+special ink for pixel-art.
+
+---
+
+**SEE ALSO**
+
+[Drawing](drawing.md) |
+[Shading](shading.md)

shading.md

@@ -31,4 +31,5 @@ Or you can use right click to move colors to the right:
 
 **SEE ALSO**
 
-[Drawing](drawing.md)
+[Drawing](drawing.md) |
+[Ink](ink.md)