dcf.cymru stimulus sheet · Art · Year 10
The photographer, the photograph and its brightness values are invented for this lesson. The tools, the menus and the rules for blend modes are real, and work the same way in most image editors.
A Year 10 pupil photographed a winding wheel memorial on a hillside above a Valleys town at sunset. Many former mining towns in South Wales keep a pit wheel as a memorial. The pupil turned the photograph to greyscale and wants to experiment with it digitally before making a print.
The pupil’s intention: “I want the wheel and its frame to be a strong, dark silhouette against a glowing sunset, like a screen print. The pit is closed, but it still stands over the houses. I want people to see both the wheel and the lit windows of the terrace below it.”
The photograph on your worksheet has been shrunk to 14 by 10 pixels so that you can work on it by hand. A real phone photograph has millions of pixels. A 4000 by 3000 pixel photograph has 12,000,000.
A digital image is a grid of pixels. In a greyscale image, each pixel stores one number for its brightness. In most images this number goes from 0 (black) to 255 (white), which gives 256 possible tones. The red lines below split the scale into the four bands used on your worksheet.
A colour image works the same way, but each pixel stores three numbers: one each for red, green and blue.
Posterise cuts the number of tones down to a few flat ones. With 4 levels, every pixel is moved into one of four bands, and every pixel in a band becomes the same colour. Smooth gradients turn into sharp-edged steps, like a poster or a screen print.
Threshold is the most extreme version: only 2 tones. You choose a threshold value. Every pixel darker than it becomes black and every pixel at or above it becomes white.
A 4 by 4 example. Left: the brightness values. Middle: posterised to 4 levels using the bands above. Right: threshold at 128.
| 22 | 95 | 150 | 231 |
| 60 | 128 | 190 | 245 |
| 64 | 127 | 201 | 110 |
| 15 | 180 | 86 | 250 |
| A | B | C | D |
| A | C | C | D |
| B | B | D | B |
| A | C | B | D |
| black | black | white | white |
| black | white | white | white |
| black | black | white | black |
| black | white | black | white |
Look at the first column. 22 and 60 are both band A, 15 is band A too, but 64 is band B. In the threshold version all four are black: the difference between them has gone.
Where to find the tools. Menus vary by version, but in GIMP both are in the Colours menu, and in Photoshop and Photopea both are under Image > Adjustments. Real software can place the band edges slightly differently from this sheet, but the idea is the same.
Most image editors let you build a picture from a stack of layers, like sheets of tracing paper. The top layer is at the top of the list.
| Blend mode | What it does to the layers below | Good for |
|---|---|---|
| Normal | The layer covers everything below it where it is solid. | Text, or a shape that must sit on top. |
| Multiply | Darkens. White on this layer disappears; dark marks darken whatever is below. | Putting a pencil or ink drawing scanned on white paper over a photograph. |
| Screen | Lightens. Black on this layer disappears; light areas lighten whatever is below. | Adding glow, or light marks on a dark background. |
| Overlay | Adds contrast. Dark areas get darker and light areas get lighter. | Adding a texture, such as a rubbing, without hiding the image. |
Opacity sets how solid a layer is, from 0% (invisible) to 100% (fully solid). Lowering the opacity of a texture layer to 30% to 50% often makes it subtle.
Work non-destructively. Keep the original photograph on its own layer, hidden or at the bottom, and work on copies or on adjustment layers. You can always go back, compare, and show the development in your portfolio.