A T.V. special effects generator which includes a store for a video image and a coefficient processor connected to the store which synthesises picture points from the stored video image to provide a processed image which has a shape dependent on mask data controlling this picture point synthesis which results in an image distorted into the desired shape to give a fish-eye effect for example. The mask data may be generated in a computer or by using a T.V. camera and the processed mask information may be used as the key input in a studio mixer. Additional control of the size of the image can be effected to provide picture zoom.