Conditional Gradient Coloring

Question

Please consider:

Manipulate[
Row[{
Graphics[Disk[]], 
Graphics[{
 Polygon[{{0, 0}, {3, 0}, {3, 1}, {0, 1}},
 VertexColors -> {White, Blend[{White, Blue}], 
 Blend[{White, Blue}], White}],
 Black, Thick,
 Line[{{i, 0}, {i, 1}}]}, ImageSize -> 300]}], 
{i, 0, 3}]

Using Szabolcs on Gradient Filling

How can I color a disc with a color below the black line?

Answer 1

Here is one solution that works because the color is on the left Whiteand the gradient is linear.

With[{max = 3, color = Blend[{White, Blue}]}, 
 Manipulate[
  Row[{Graphics[{Opacity[i/max], color, Disk[]}], 
    Graphics[{Polygon[{{0, 0}, {max, 0}, {max, 1}, {0, 1}}, 
       VertexColors -> {White, color, color, White}], Black, Thick, 
      Line[{{i, 0}, {i, 1}}]}, ImageSize -> 300]}], {i, 0, max}]]

---

If you had two different colors for each end (i.e. something other than White), the approach Opacitywill not work. Instead, you can use the optional mix fraction argument for the Blendcolors in the desired proportion. Here is an example:

With[{max = 3, color1 = Red, color2 = Green}, 
 Manipulate[
  Row[{Graphics[{Blend[{color1, color2}, i/max], Disk[]}], 
    Graphics[{Polygon[{{0, 0}, {max, 0}, {max, 1}, {0, 1}}, 
       VertexColors -> {color1, color2, color2, color1}], Black, 
      Thick, Line[{{i, 0}, {i, 1}}]}, ImageSize -> 300]}], {i, 0, 
   max}]]

Answer 2

, - , Opacity . Szabolcs, Blend :

skyBlue = Blend[{White,Blue}];
Manipulate[ Row[{ Graphics[{Blend[{White,skyBlue},i/3], Disk[]}],  
 Graphics[{  Polygon[{{0, 0}, {3, 0}, {3, 1}, {0, 1}},  
 VertexColors -> {White, skyBlue,   
 skyBlue, White}],  Black, Thick,  
 Line[{{i, 0}, {i, 1}}]}, ImageSize -> 300]}],  {i, 0, 3}]

i 3, 0 1.