The enableiOES() method of the OES_draw_buffers_indexed WebGL extension enables blending for a particular draw buffer.
enableiOES(target, index)targetMust be gl.BLEND.
indexAn integer i specifying the draw buffer associated with the constant gl.DRAW_BUFFERi, see WebGL draw buffer constants.
None (undefined).
target is not gl.BLEND, a gl.INVALID_ENUM error is thrown.index is not a valid value, a gl.INVALID_VALUE error is thrown.The following two calls enable blending for the draw buffers gl.DRAW_BUFFER0 and gl.DRAW_BUFFER1.
const ext = gl.getExtension("OES_draw_buffers_indexed");
ext.enableiOES(gl.BLEND, 0);
ext.enableiOES(gl.BLEND, 1);You can use WebGLRenderingContext.getParameter() to see how many draw buffers are available.
const maxDrawBuffers = gl.getParameter(gl.MAX_DRAW_BUFFERS);