Mookerjee, in his book Yoga Art, describes the translation of sound and vibration into mental images through meditational practice. He explains these images and yogic symbols with various colours depending on the vibration.
We see examples of the mental visuals one experiences during tantric meditation in these images. Some of which include different shapes also denoting female and male principles; triangles symbolise the yoni (the female sexual organ), whilst the circle is representative of the universe, holding both male and female principles. There are also associations to manifestation, expansion and creation.
Plate 44- The Origin of the Universe. Mount Meru the golden Womb, and the stars, float against a background of red which signifies atomic particles- Rajasthan, 18th Century
Plate 56- Mandala from an illustrated manuscript page. Symbol of the evolving and dissolving universe, tension and rest. The principle of the circle has led to the enneagram. This, in yoga philosophy, is a symbol of great potency. Rajasthan, 17th Century