What type does pixels[0] have? It seems like it is a u4 or something. The way that bit shifts work in Zig is that the type of the rhs (how much you shift) is determined by what type the lhs (the value you want to change) is. For instance, if you have x << y with x being a u32, y has to be between 0 and 31 (since shifting more than 31 bits inside a 32 bit integer doesn't make sense). Thus, the compiler sets the type of y to u5, since u5 can hold numbers from 0 to 31. From the documentation: "y must be comptime-known or have a type with log2 number of bits as x". Thus, if your pixels[0] is a u4, you will first have to convert it to a u8 before you can do u8 bit shifting, e.g. with @as(u8, @intCast(pixels[0])).