Scratch provides two blocks to perform color detection on the screen: <touching color (#ff0000)?> and <color (#00ff00) is touching (#ff0000)?>. These blocks return true when the sprite is touching the specified color on the screen. There is no block for directly reading the color of a pixel on the screen but it has been suggested.[1] The workaround is to check many different colors until a match is found, which this page will describe.
Uses
Color detection has a few uses:
- A color picker in a paint editor project
- Barcode or QR code reader projects
- Loading raster textures to use in pen-based 3D projects
- Encoding data as colors in a costume to avoid storing it in project.json (which is more restrictive on file size).
Programming
Basic Script
Scratch uses 24-bit color so a naïve implementation would be to loop over all 16 million colors:
define get color // run without screen refresh
set [color v] to [0] // the result will be stored in this variable
repeat (16777215) // 256^3-1=16777215, the total number of colors possible minus one
if <touching color (color)?> then // if it finds the color, the script stops
stop [this script v]
end
change [color v] by (1) // increments the color each time through the loop
end
 Tip: | A custom block set to "run without screen refresh" will eliminate the delay caused by loops. It is unlikely to make this script instant but it will be a lot faster. |
Color Sensitivity
The color detection blocks are unable to detect all 16 million colors. They only sense 16 different shades of blue and 32 different shades of red and green, allowing the loop iterations to be decreased considerably:
define get color // run without screen refresh
set [color v] to [0] // the result will be stored in this variable
repeat (32)
repeat (32)
repeat (16)
if <touching color (color)?> then
stop [this script v]
end
change [color v] by (16) // cycles through shades of blue
end
change [color v] by (1792) // cycles through shades of green
end
change [color v] by (458752) // cycles through shades of red
end
Binary Search
A further improvement can be made with the use of a binary search. By darkening the screen with a translucent sprite or pen, the colors can be adjusted in a way that allows them to be found in fewer guesses. See for example Binary Search Image Scanner.
