The enableiOES() method of the OES_draw_buffers_indexed WebGL extension enables blending for a particular draw buffer.

target

Must be gl.BLEND.

index

An integer i specifying the draw buffer associated with the constant gl.DRAW_BUFFERi, see WebGL draw buffer constants.

None (undefined).

• If target is not gl.BLEND, a gl.INVALID_ENUM error is thrown.
• If 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);

• OES_draw_buffers_indexed.disableiOES()
• WebGL draw buffer constants