Optical illusion

This optical illusion is going under the hammer at Christies in London on September 3 with an estimate of £600-800.

The catalogue description reads:

A MASONIC LODGE CANVAS WALL-HANGING LATE 20TH CENTURY
Of rectangular form, with white and black chequering within a red and blue triangular border, together with a grey storage tube with type-written labels 'SUB URBAN CHAPTER OF INSTRUCTION'
121 ¾ in. x 62 in. (309 x 158 cm.)