All competition entries will be displayed on-screen, even if the competition is about printed images and the judge will only be looking at your prints. After all, everyone would like to see the images, make up their own mind, and see if the judge agrees with them! It therefore makes sense to consider how images are rendered, and to ensure that they will always look their best.

The rendering of images is all about pixels. The standard way of showing an image is to ensure that it fits on the display, which usually means that in one direction all the display’s pixels are used and in the other the image has some unused pixels on either side. However, the number of pixels in image and display rarely matches and the colour and intensity of each display pixel is obtained by interpolating the pixels of the image.

This can give rise to an artificial pattern of lighter and darker areas in the displayed image; it happens most often when there are lines in the image itself which are almost but not quite aligned with the array of pixels in the display. This is shown below where the same image is displayed twice, but with a different number of pixels in horizontal and vertical direction.

You can probably see that at least one of the images displays some strange interference, or Moiré pattern (usually the one on the left-hand side). If not, you will be able to make it happen by either increasing or decreasing the size of this page in your browser. In Internet Explorer an increase is achieved by simultaneously pressing the Ctrl and the + key on your keyboard, while a decrease requires Ctrl and –; in other web browsers you may need to use a different command.

Clearly, you don’t want to rely on the vagaries of the mapping being OK on the night: it simply has to be perfect! The only way this can be guaranteed is by a one to one mapping, where the number of pixels in your image fits exactly on the display. Images smaller than the display are already mapped one to one by the rendering process which never enlarges them, but in practice all competition entries will need to be resized first to ensure one to one mapping. Digital cameras can easily produce images of more than 3000 pixels in either direction, many more than are present on the average display.

The club’s laptop has 1080 pixels in the vertical direction, and this explains why the height of an image cannot exceed 1080 pixels. Normally, the width is not allowed to exceed 1440 pixels as this gives a pleasing aspect ratio of 4×3. However, the laptop can display more pixels horizontally and in the “Chairman’s Choice” competition the maximum size can be 1920×1080 pixels, while for the “Panoramic” competition it is 1920×640 pixels. Note that the first number always denotes the horizontal, and the second the vertical dimension.

In general images in landscape orientation can occupy a greater maximum display area (1440×1080) than images in portrait orientation (1080×1080, which strictly speaking is a square format). For this reason some people will not use portrait orientation for a “Projected Image” competition. They prefer to use these images in a “Printed Image” competition because it is the mounted, and not the projected, image which will be judged. There are no set rules for this and the choice is yours.