Comment by nightcracker

    QOI_DIFF24 {
        u8 tag  :  4;   // b1110
        u8 dr   :  5;   // 5-bit   red channel difference: -15..16
        u8 dg   :  5;   // 5-bit green channel difference: -15..16
        u8 db   :  5;   // 5-bit  blue channel difference: -15..16
        u8 da   :  5;   // 5-bit alpha channel difference: -15..16
    }

It bothers me more than it should that the 5-bit signed differences aren't -16..15 matching two's complement but -15..16 instead.