A computer implemented method for controlling the display of a computer device to display randomly selected multiple tile images adjacent to each other to form a background image, the method comprising; storing in a computer memory at the computer device, image texture data, the image texture data comprising a plurality of sets of predefined masks, each set of predefined masks for forming a respective tile image comprising multiple portions, wherein each portion comprises a rectangle having a long side and a short side being a vertical height in the image, the rectangles in a tile image defining strata for composing the image wherein each mask,, of the set determines how the respective portion of the tile image is to be rendered, wherein the image texture data for predefined masks for rendering a portion of the tile image is stored in a respective channel of an image file, wherein each channel stores a plurality of predefined masks for creating that portion of the image, and wherein the set of predefined masks for rendering each tile image comprises a mask from each channel; selecting at random multiple tile images for display from a plurality of tileimages; determining the location where each mask,, in the set of predefined masks for forming each selected tile image is stored in the image texturedata; supplying an indication of said location to a shader program executed on the computer device to control the shader program to use the set of predefined masks to form each selected tile image; selecting a vertical scale adjustment parameter at random, and using the vertical scale adjustment parameter to adjust the vertical height of a rectangle onto which the predefined masks are to be mapped by the shader program prior to supply of the rectangle to the shader program, or supplying the vertical scale adjustment parameter to the shader program to adjust the vertical height of a rectangle onto which the predefined masks are to be mapped by the shader program; determining at random that at least one of the tile images is to be flipped horizontally prior to display, and controlling the shader program to flip the tile image prior to display based on said determination; and displaying the randomly selected tile images adjacent to each other on the display to form the background image. storing in a computer memory at the computer device, image texture data, the image texture data comprising a plurality of sets of predefined masks, each set of predefined masks for forming a respective tile image comprising multiple portions, wherein each portion comprises a rectangle having a long side and a short side being a vertical height in the image, the rectangles in a tile image defining strata for composing the image wherein each mask,, of the set determines how the respective portion of the tile image is to be rendered, wherein the image texture data for predefined masks for rendering a portion of the tile image is stored in a respective channel of an image file, wherein each channel stores a plurality of predefined masks for creating that portion of the image, and wherein the set of predefined masks for rendering each tile image comprises a mask from each channel; selecting at random multiple tile images for display from a plurality of tileimages; determining the location where each mask,, in the set of predefined masks for forming each selected tile image is stored in the image texturedata; supplying an indication of said location to a shader program executed on the computer device to control the shader program to use the set of predefined masks to form each selected tile image; selecting a vertical scale adjustment parameter at random, and using the vertical scale adjustment parameter to adjust the vertical height of a rectangle onto which the predefined masks are to be mapped by the shader program prior to supply of the rectangle to the shader program, or supplying the vertical scale adjustment parameter to the shader program to adjust the vertical height of a rectangle onto which the predefined masks are to be mapped by the shader program; determining at random that at least one of the tile images is to be flipped horizontally prior to display, and controlling the shader program to flip the tile image prior to display based on said determination; and displaying the randomly selected tile images adjacent to each other on the display to form the background image.