Clarified level = 0 default parameter code

build/three.module.js

@@ -26319,6 +26319,8 @@ function WebGLRenderer( parameters ) {
 
 	this.copyTextureToTexture = function ( position, srcTexture, dstTexture, level ) {
 
+		if ( level === undefined ) level = 0;
+
 		var width = srcTexture.image.width;
 		var height = srcTexture.image.height;
 		var glFormat = utils.convert( dstTexture.format );
@@ -26328,14 +26330,16 @@ function WebGLRenderer( parameters ) {
 
 		if ( srcTexture.isDataTexture ) {
 
-			_gl.texSubImage2D( 3553, level || 0, position.x, position.y, width, height, glFormat, glType, srcTexture.image.data );
+			_gl.texSubImage2D( 3553, level, position.x, position.y, width, height, glFormat, glType, srcTexture.image.data );
 
 		} else {
 
-			_gl.texSubImage2D( 3553, level || 0, position.x, position.y, glFormat, glType, srcTexture.image );
+			_gl.texSubImage2D( 3553, level, position.x, position.y, glFormat, glType, srcTexture.image );
 
 		}
 
+		if ( level === 0 && dstTexture.generateMipmaps ) _gl.generateMipmap( 3553 );
+
 		state.unbindTexture();
 
 	};

src/renderers/WebGLRenderer.js

@@ -2803,7 +2803,7 @@ function WebGLRenderer( parameters ) {
 
 	this.copyTextureToTexture = function ( position, srcTexture, dstTexture, level ) {
 
-		level = level || 0;
+		if ( level === undefined ) level = 0;
 
 		var width = srcTexture.image.width;
 		var height = srcTexture.image.height;
@@ -2822,7 +2822,7 @@ function WebGLRenderer( parameters ) {
 
 		}
 
-		if ( level == 0 && dstTexture.generateMipmaps ) _gl.generateMipmap( _gl.TEXTURE_2D );
+		if ( level === 0 && dstTexture.generateMipmaps ) _gl.generateMipmap( _gl.TEXTURE_2D );
 
 		state.unbindTexture();