javascript - Three.js Why renderer with viewport & scissor and I have a complete black canvas -

i have typical ui - 2 vertical windows. left canvas & renderer, right ui.

inside ui have special viewers, example simple top view window.

i have used viewports && scissors approach. works fine.

the question miss special parameter avoid complete canvas black-filled covers have (my ui) behind. expected viewport & scissors going let me have 'clean' areas (not filled) inside canvas. not....

the alpha approach ( renderer = new three.webglrenderer( { alpha: true } );) fix don't know if best solution (what performance) or there way program (enhancement) related viewport & scissors.

if i'm going use viewports there no sense fill black.

(also main question ... if have viewport & scissor, why , fill canvas black ?)

it totally okay use this:

var renderer = new three.webglrenderer({ alpha: true });  

it common approach can read here in answer

you can change color this:

renderer.setclearcolor( 0xffffff );