Newbie Keeps Losing Alpha Values In Canvas

I'm probably doing something boneheaded here but I'm having difficulties getting alpha values to cooperate in Canvas. I'm trying to sample an opaque color from a spot on the canva

Solution 1:

Use context.globalAlpha instead of using rgba fill:

p = ground.ctx.getImageData(loc.x, loc.y, 1, 1).data; 
col = {R: p[0], G: p[1], B: p[2], a: p[3]};

// note: globalAlpha uses a scale of 0-1
// and getImageData uses a scale of 0-255
ground.ctx.globalAlpha = a/255-.1;

ground.ctx.fillStyle = 'rgb(' + col.R + ', ' + col.G + ', ' + col.B + ')';
ground.ctx.fillRect(nuLoc.x, nuLoc.y, sqrSize, sqrSize);

// reset globalAlpha when you're done
ground.ctx.globalAlpha = 1;