Using still images in video
- — 22 March, 2004 07:29
In this column we look at how to use still images to enhance your video project, as well as how to prepare images from sources such as scanned photos and digital still cameras.
There are many ways to incorporate still photos into a video, the most obvious being to display a still for an intended duration instead of a video clip. You can do many other things, such as pan and/or zoom over the image to give an impression of motion, use a picture-in-picture effect of a small still inserted into a video, or create a moving montage of several stills.
Getting your image
Still images can be sourced from a variety of areas including stock photography CDs, digital still cameras and scanned photos or 35mm film. When working with scanned images, high resolution is not crucial; even a 1Mp camera can provide more than enough resolution to use in a video project. The most important part of the initial preparation is to get a clean image with good colour contrast.
Preparing your image
Unlike images grabbed as a frame from video footage, most digital still images from non-video sources need to be processed to some degree. An important issue to resolve when preparing your image for video is the aspect ratio. Most video is displayed with an aspect ratio of 4:3. For example, if your image is 16cm wide, the height of the image needs to be 12cm. This will ensure that the image takes up the whole area on a standard digital video screen. There are exceptions to this rule, such as when working with wide screen video, where the ratio needs to be 16:9.
One of the advantages of using an image from a digital still camera is that most models use the DV-friendly aspect ratio of 4:3, making the task of preparing an image for video much easier. However, cropping your digital still image is recommended in the preparation process, as this will give you a good idea as to what part of the image will be displayed.
PC screen or TV monitor
Images are always displayed on a PC screen using square pixels; therefore, all image file formats use square pixels. This means that if an image measures 640x480 pixels, it will display at an aspect ratio of 4:3 on a PC screen.
The same is not always true of video frames, par-ticularly when shown on a standard analog video monitor. For example the DV format is 720x576 for PAL, but the final image when rendered on a TV is 4:3. When a still image with square pixels is incorporated, the different pixel shape must be taken into account or the image will be distorted when shown on a TV.
Therefore, when working in a graphics program (such as Adobe Photoshop) with an image which will be imported into a video editing program, the square pixels should be scaled to non-square pixels for video encoding.
For more information on setting the appropriate resolution for your images when working with video for playback on a DVD or VCD, as well as tips on setting up a background colour to compensate for different image shapes, check out Scott Mendham’s Here’s How Graphics article, Photos for TV and DV.
Adobe’s new Photoshop CS has a handy new feature to help convert images to a more video friendly format. Select the image you want to convert and go to Image-Pixel Aspect Ratio. You will notice that the choices available include DV PAL, Widescreen and Anamorphic. Your image will probably be stretched (slightly wider on normal DV and more pronounced on Widescreen) when converting to any of the video-friendly aspect ratios, so take this into account when choosing the type of images to include in your video.
Still effects
The ability to perform effects working with still images depends largely on the capabilities of your video editing program. All allow you to import a still image, and most programs allow you to alter the duration of the clip and whether to fade in/out of the clip or have it enter and exit from a certain point of the screen.
If you want to add a special element to the still image, such as a panning or zoom effect, it is important to keep the resolution of the image as high as possible. This is due to the way the movie reveals only small portions of an image over time, thus giving the impression that the viewer is actually moving his/her gaze across an image. This effect, often called the ‘Ken Burns Effect’, can be particularly useful when dealing with a landscape still or a shot with a gathering of people. Alternatively, working with small images panning across a black video screen offers an interesting effect when dealing with several different digital still images within a sequence.
