Digital Graphics and Animation Planning and Design

Digital processing and editing techniques

Digital images can be edited in limitless ways. Powerful editing and processing facilities in software packages such as Photoshop, Animate, GIMP and many others allow users to:

• Layer parts of images together to create composites (using parts of different images together to create one final image)
• Modify the scale of parts of images
• Alter coloration of images (for example, create black and white images from colour, use AI to create colour images from black and white)
• Convert from vector images to raster images (and software is available to ‘trace’ around raster images and create vectors from them
• Wishing animation, video can be processed to alter frame rate, or add ‘film style’ effects

This list is far from exhaustive! For a detailed list, see here (image editing) and here (video editing).

Compression formats and techniques

All compression algorithms aim to reduce the amount of data required to represent something.

In some cases, lossless compression is required – for example, documents and software would become useless if compression resulted in data loss.

For data describing images, audio and video, the use case will determine whether lossless or lossy compression is most appropriate.

Image Compression

Images are commonly compressed using both types of compression. Where lossy compression is used, the goal is to remove what is termed as ‘unnecessary detail’ – that is, information that is either impossible to discern, or hard to discern, with the naked eye. By reducing the complexity of the image, the resulting (simpler) data can be compressed. The more detail that is sacrificed, the simpler the data becomes, and the greater the degree of compression that can be achieved. Some examples, along with use-cases are below:

File format	Type of compression	Use-case
JPEG	Lossy – quality can be varied according to need	Digital camera images (high quality) Images for use on the Internet (lower quality)
BMP	Lossless	Rarely used, as file sizes for images are huge
PNG	Lossless	Used in editing as supports transparency and therefore amenable to layering Used on web where transparency is required (e.g. non-rectangular images overlaid on a background)
TIFF	Lossy or lossless at user’s choice	Rarely used as a lossy format (JPEG more widely supported) Used as a lossless (often uncompressed) format for high-quality print and creating image masters that may go on to be further edited in future

Video Compression

Video is simply a stream of images that update at rapid, regular intervals. Consider a lossless image of a park. There might be a lot of sky, trees, grass, a path, and a few people there.

Now consider a video of this same scene. In the course of a couple of seconds (50 frames in UK TV, or 50 images), most of the sky will remain unchanged. The trees won’t be moving anywhere. The grass won’t have grown noticeably. The people may have moved a bit. Mostly, images will be very similar to the one preceding it.

Video compression works on this basis. You take a keyframe, and encode this image. For the next, say 10, frames, you only record the differences between successive images. Of course, this process isn’t perfect, so at regular intervals (in this example, after 10 frames), a new keyframe is included as reference for the next 10 frames.

The quality of encoded video is affected by the bitrate – how many bits are used per second to encode the video. The lower this value, the fewer keyframes you can use, and the resulting video will include more artefacts. (Bits of an image that don’t disappear for a while, or blockiness in moving areas).

While editing for commercial purposes, it is common to use uncompressed video; normally, home users will use compressed video. Benefits of using compressed video include:

• Accessible to home users
• Does not require expensive and powerful hardware
• Storage requirements are realistic

The primary disadvantage of using compressed video is that the video will have been compressed once (when created), decompressed during editing to view, and then recompressed when exported as a final product. Every compression cycle will introduce artefacts to the video.

Video formats are far more complex than image formats: while a JPEG image will use the JPEG compression format, an MP4 video file can use one of many different video formats (codecs – enCOder/DECoder), and one of many audio formats (codecs). This is of greatest importance at the export stage, as the video format and codecs chosen will affect the range of devices on which the video can be displayed.

Quality metrics

A quality metric is a way of defining whether a product is fit for purpose and of ‘good enough’ quality. In order to state that something has met the required quality, you must specify the how the quality will be measured, and quantify what level must be achieved. Note, this means that “it looks good” is not an appropriate justification or target.

Some areas which can be assessed when determining the quality of a product include:

• Image quality (for example, file size, compression)
• Compatibility (on what devices should the product work)
• User experience (did it help users understand the product)
• Usability (if creating icons or guidance, was it clear)