Clamping, scaling, format conversion

Got to spend a few hours poking at the photo-effects app i'm doing in conjunction with 'ffts'. I ended up having to use some NEON for performance.

One interesting solution along the way was code that took 2x2-channel float sequences (i.e. 2xcomplex number arrays) and re-wound them back to 4-channel bytes, including scaling and clamping.

I utilised the fixed-point variant of the VCVT instruction which performs the scaling to 8 bits with clamping below 0. For the high bits I used the saturating VQMOVN variant of move with narrow.

I haven't run it through the cycle counter (or looked the details up) so it could probably do with some jiggling or widening to 32 bytes/iteration but the current main loop is below.

        vld1.32         { d0[], d1[] }, [sp]

        vld1.32         { d16-d19 },[r0]!
        vld1.32         { d20-d23 },[r1]!     
1:
        vmul.f32        q12,q8,q0               @ scale
        vmul.f32        q13,q9,q0
        vmul.f32        q14,q10,q0
        vmul.f32        q15,q11,q0

        vld1.32         { d16-d19 },[r0]!       @ pre-load next iteration
        vld1.32         { d20-d23 },[r1]!

        vcvt.u32.f32    q12,q12,#8              @ to int + clamp lower in one step
        vcvt.u32.f32    q13,q13,#8
        vcvt.u32.f32    q14,q14,#8
        vcvt.u32.f32    q15,q15,#8

        vqmovn.u32      d24,q12                 @ to short, clamp upper
        vqmovn.u32      d25,q13
        vqmovn.u32      d26,q14
        vqmovn.u32      d27,q15

        vqmovn.u16      d24,q12                 @ to byte, clamp upper
        vqmovn.u16      d25,q13

        vst2.16         { d24,d25 },[r3]!

        subs    r12,#1
        bhi     1b

The loading of all elements of q0 from the stack was the first time I've done this:

        vld1.32         { d0[], d1[] }, [sp]

Last time I did this I thing I did a load to a single-point register or an ARM register then moved it across, and I thought that was unnecessarily clumsy. It isn't terribly obvious from the manual how the various versions of VLD1 differentiate themselves unless you look closely at the register lists. d0[],d1[] loads a single 32-bit value to every lane of the two registers, or all lanes of q0.

The VST2 line:

        vst2.16         { d24,d25 },[r3]!

Performs a neat trick of shuffling the 8-bit values back in to the correct order - although it relies on the machine operating in little-endian mode.

The data flow is something like this:

 input bytes:        ABCD ABCD ABCD
 float AB channel:   AAAA BBBB AAAA BBBB
 float CD channel:   CCCC DDDD CCCC DDDD   
 output bytes:       ABCD ABCD ABCD

As the process of performing a forward then inverse FFT ends up scaling the result by the number of elements (i.e. *(width*height)) the output stage requires scaling by 1/(width*height) anyway. But this routine requires further scaling by (1/255) so that the fixed-point 8-bit conversion works and is performed 'for free' using the same multiplies.

This is the kind of stuff that is much faster in NEON than C, and compilers are a long way from doing it automatically.

The loop in C would be something like:

float clampf(float v, float l, float u) {
   return v < l ? l : (v < u ? v : u);
}

    complex float *a;
    complex float *b;
    uint8_t *d;
    float scale = 1.0f / (width * height);
    for (int i=0;i<width;i++) {
       complex float A = a[i] * scale;
       complex float B = b[i] * scale;

       float are = clampf(creal(A), 0, 255);
       float aim = clampf(cimag(A), 0, 255);
       float bre = clampf(creal(B), 0, 255);
       float bim = clampf(cimag(B), 0, 255);

       d[i*4+0] = (uint8_t)are;
       d[i*4+1] = (uint8_t)aim;
       d[i*4+2] = (uint8_t)bre;
       d[i*4+3] = (uint8_t)bim;
    }

And it's interesting to me that the NEON isn't much bulkier than the C - despite performing 4x the amount of work per loop.

I setup a github account today - which was a bit of a pain as it doesn't work properly with my main browser machine - but I haven't put anything there yet. I want to bed down the basic data flow and user-interaction first.