When you need to draw an intercection of two entities, e.g. a reflection in the mirror, you can test against the stencil buffer, that has to be enabled with stencil option in toHtmlWith.

Stencil test decides if the pixel should be drawn on the screen. Depending on the results, it performs one of the following operations on the stencil buffer:

• fail—the operation to use when the stencil test fails;
• zfail—the operation to use when the stencil test passes, but the depth test fails;
• zpass—the operation to use when both the stencil test and the depth test pass, or when the stencil test passes and there is no depth buffer or depth testing is disabled.

For example, draw the mirror Entity on the screen and fill the stencil buffer with all 1's:

test
    { ref = 1
    , mask = 0xFF
    , test = always    -- pass for each pixel
    , fail = keep      -- noop
    , zfail = keep     -- noop
    , zpass = replace  -- write ref to the stencil buffer
    , writeMask = 0xFF -- enable all stencil bits for writing
    }

Crop the reflection Entity using the values from the stencil buffer:

test
    { ref = 1
    , mask = 0xFF
    , test = equal  -- pass when the stencil value is equal to ref = 1
    , fail = keep   -- noop
    , zfail = keep  -- noop
    , zpass = keep  -- noop
    , writeMask = 0 -- disable writing to the stencil buffer
    }

You can see the complete example here.

The Test allows you to define how to compare the reference value with the stencil buffer value, in order to set the conditions under which the pixel will be drawn.

always         -- Always pass
equal          -- ref & mask == stencil & mask
never          -- Never pass
less           -- ref & mask < stencil & mask
greater        -- ref & mask > stencil & mask
notEqual       -- ref & mask != stencil & mask
lessOrEqual    -- ref & mask <= stencil & mask
greaterOrEqual -- ref & mask >= stencil & mask

Defines how to update the value in the stencil buffer.

Sets the stencil buffer value to ref from the stencil test.

Keeps the current stencil buffer value. Use this as a noop.

Sets the stencil buffer value to 0.

Increments the current stencil buffer value. Clamps to the maximum representable unsigned value.

Decrements the current stencil buffer value. Clamps to 0.

Bitwise inverts the current stencil buffer value.

Increments the current stencil buffer value. Wraps stencil buffer value to zero when incrementing the maximum representable unsigned value.

Decrements the current stencil buffer value. Wraps stencil buffer value to the maximum representable unsigned value when decrementing a stencil buffer value of zero.

Different options for front and back facing polygons.