Question

I’m very new to C and I’m writing a Ruby C Extension. One of the functions should compute a bland between two colours. The Color struct uses char to store the RGB values. The blending ratio is a double between 0.0 and 1.0.

Is it bad practice to mix char and double in an operation I have done here? Could there be potential problems?

I’m guessing that if weight is not between 0.0 and 1.0 the could be problems as it might result in a value smaller than 0 or larger than 255.

Should I be explicitly casting types?

typedef struct Color    Color;

static struct Color {
  unsigned char red, green, blue, alpha;
};


static Color
color_blend(  Color color1, Color color2, double weight )
{
  Color color3 = { 0, 0, 0, 0 };

  color3.red    = ( 1 - weight ) * color1.red   + ( weight * color2.red );
  color3.green  = ( 1 - weight ) * color1.green + ( weight * color2.green );
  color3.blue   = ( 1 - weight ) * color1.blue  + ( weight * color2.blue );
  color3.alpha  = ( 1 - weight ) * color1.alpha + ( weight * color2.alpha );

  return color3;
}

Answer 1

You don’t need to perform explicit casting; the compiler’s implicit casting should do the same thing. However, there are two good reasons to consider explicit casts:

1. On higher warning levels, you may get warning messages about loss of precision etc. for implicit casts.
2. Explicit casts help document your intent to readers of your code.

I don’t think there will be any range issues with your code. However, you might consider adding round-to-nearest behaviour, as currently a computed value of e.g. 99.999 will truncate to 99.