Answer:
valid means it good and invalid means bad
The process of oil painting is using drying oil with pigment in order to create a picture. The person who is most commonly known as the inventor of the technique is Van Eyck. Linseed oil, poppy seed oil, walnut oil and safflower oil are the most commonly used oils for this technique. By the peak point of the Renaissance, the most common painting technique was oil painting in comparison to tempera paints. Oil paints became more popular than tempera due to their ability to take longer to dry providing artist more time to paint as well as being brighter and providing a smooth finish in comparison to tempera.
They can be used to adjust the bending angle by reference, stroke measurement or angle measurement.
not sure if this is it but i hope it helps
