Expression with still images
A still image is a picture that stays still: a photo, a drawing, a poster or a chart. With one picture we can show a feeling or an idea.
- Framing: what we put inside the picture and what we leave out.
- Composition: where things sit. The rule of thirds splits the picture into 3 × 3 parts. Putting the main subject on a crossing of the lines feels balanced.
- Colour: warm colours (red, yellow) feel lively; cool colours (blue, green) feel calm.
- Light: a bright picture feels happy; a dark one feels serious or sad.
- Angle and size: a close-up shows detail; a wide shot shows the place.
How a digital still image is stored
A digital picture is a grid of tiny squares called pixels. Each pixel stores one colour as three numbers: red, green and blue (RGB), each from 0 to 255. A picture 16 × 16 has 256 pixels.
Resolution is how many pixels the picture has, for example 1600 × 1200. More pixels give more detail but a bigger file. If we stretch a small picture, it turns blocky. Megapixel = one million pixels.
There are two kinds of image. A raster image is made of pixels (photos). A vector image is made of lines and shapes written as maths, so it stays sharp at any size (logos).
Editing still images
Editing means changing a picture after it is made. The common jobs are:
- Crop: cut away the edges to improve the frame. The pixel count becomes smaller.
- Resize: change the number of pixels, for example to fit a web page.
- Brightness and contrast: make all pixels lighter or darker, or make light and dark parts more different.
- Colour correction: fix a picture that looks too yellow or too blue.
- Retouch: remove a spot or a stray object.
- Layers: keep text, shapes and the photo on separate sheets, so one can be changed without touching the others.
Good habit: always edit a copy and keep the original.
Producing still-image content
Making a finished picture follows steps: (1) decide the purpose and audience; (2) capture or draw the picture; (3) edit it; (4) add text or graphics if needed; (5) save in the right format; (6) publish it and check how people react.
File formats: JPEG is small and good for photos but loses a little detail each time it is saved (lossy). PNG keeps every pixel (lossless) and can be transparent, so it suits logos and screenshots. GIF has few colours and can animate. RAW is the untouched data from a camera.
Print needs more pixels (about 300 per inch) than a screen (about 72 to 150). Always give credit and respect copyright when you use someone else's picture.
Key formulas and definitions
- Number of pixels = width × height
- Megapixels = (width × height) ÷ 1,000,000
- Uncompressed size (bytes) = width × height × 3 (24-bit colour)
- Print size (inches) = pixels ÷ pixels per inch (ppi)
Worked examples
1. A photo is 1600 × 1200 pixels. How many pixels and how many megapixels?
Pixels = 1600 × 1200 = 1,920,000. Megapixels = 1,920,000 ÷ 1,000,000 = 1.92 MP.
2. A 4000 × 3000 photo is cropped to half the width and half the height. How many pixels are left?
New size = 2000 × 1500 = 3,000,000 pixels. This is one quarter of the original 12,000,000.
3. How big is an uncompressed 24-bit picture of 1000 × 800 pixels?
Each pixel needs 3 bytes. Size = 1000 × 800 × 3 = 2,400,000 bytes, about 2.4 MB.
4. You need a 6-inch wide print at 300 ppi. How many pixels wide must the picture be?
Pixels = 6 × 300 = 1800 pixels wide. A picture with fewer pixels will look blocky when printed.
5. Which format will you pick for (a) a holiday photo to send on a chat app, (b) a school logo with a see-through background?
(a) JPEG: small file, good for photos. (b) PNG: it keeps sharp edges and allows transparency.
Common mistakes
- Making a small picture bigger and expecting more detail. Stretching only makes the pixels bigger and the picture blocky.
- Editing the original file and saving over it. Always save a copy.
- Saving a photo as JPEG again and again. Each save loses a little detail.
- Putting the subject in the exact middle every time. Try the rule of thirds for a livelier picture.