Class BlockDXTAlphaLinear

common/dds.cpp:445–514

}

struct BlockDXTAlphaLinear {
    uchar alpha0;
    uchar alpha1;
    uchar bits[6];

    void GetAlphas( uchar alpha_array[8] ) {
        alpha_array[0] = alpha0;
        alpha_array[1] = alpha1;

        // 8-alpha or 6-alpha block?
        if( alpha_array[0] > alpha_array[1] ) {
            // 8-alpha block:  derive the other 6 alphas.
            // 000 = alpha_0, 001 = alpha_1, others are interpolated

            alpha_array[2] = ( 6 * alpha0 +     alpha1) / 7;	// bit code 010
            alpha_array[3] = ( 5 * alpha0 + 2 * alpha1) / 7;	// Bit code 011
            alpha_array[4] = ( 4 * alpha0 + 3 * alpha1) / 7;	// Bit code 100
            alpha_array[5] = ( 3 * alpha0 + 4 * alpha1) / 7;	// Bit code 101
            alpha_array[6] = ( 2 * alpha0 + 5 * alpha1) / 7;	// Bit code 110
            alpha_array[7] = (     alpha0 + 6 * alpha1) / 7;	// Bit code 111
        } else {
            // 6-alpha block:  derive the other alphas.
            // 000 = alpha_0, 001 = alpha_1, others are interpolated

            alpha_array[2] = (4 * alpha0 +     alpha1) / 5;		// Bit code 010
            alpha_array[3] = (3 * alpha0 + 2 * alpha1) / 5;		// Bit code 011
            alpha_array[4] = (2 * alpha0 + 3 * alpha1) / 5;		// Bit code 100
            alpha_array[5] = (    alpha0 + 4 * alpha1) / 5;		// Bit code 101
            alpha_array[6] = 0x00;								// Bit code 110
            alpha_array[7] = 0xFF;								// Bit code 111
        }
    }

    void GetBits( uchar bit_array[16] ) {
        // Split 24 packed bits into 8 bytes, 3 bits at a time.
        uint b = bits[0] | bits[1] << 8 | bits[2] << 16;
        bit_array[0] = uchar(b & 0x07);
        b >>= 3;
        bit_array[1] = uchar(b & 0x07);
        b >>= 3;
        bit_array[2] = uchar(b & 0x07);
        b >>= 3;
        bit_array[3] = uchar(b & 0x07);
        b >>= 3;
        bit_array[4] = uchar(b & 0x07);
        b >>= 3;
        bit_array[5] = uchar(b & 0x07);
        b >>= 3;
        bit_array[6] = uchar(b & 0x07);
        b >>= 3;
        bit_array[7] = uchar(b & 0x07);

        b = bits[3] | bits[4] << 8 | bits[5] << 16;
        bit_array[8] = uchar(b & 0x07);
        b >>= 3;
        bit_array[9] = uchar(b & 0x07);
        b >>= 3;
        bit_array[10] = uchar(b & 0x07);