How do I send part of an ArrayBuffer to SPI

Finally got it to work - perhaps obvious in retrospect!

E.mapInPlace(
 new Uint8Array(img.buffer, chunks*CHUNKSIZE*img.bpp/8, remnt),  
 g.chunkbuf,  img.palette,  img.bpp
);
spi.write(new Int8Array(g.chunkbuf.buffer,0,remnt*2));