Image editing basics
FoundationBitmaps pixelate when enlarged, vectors stay smooth at any size; resolution counts pixels; crop, adjust and layer are the basic edits; JPG photos, PNG transparency, GIF animation.
- Bitmaps are pixel grids that distort when enlarged; vectors never do.
- Resolution counts pixels — sharper means bigger files.
- Three basic edits: crop, adjust, layer.
文體小知識：軟件操作課一步一步示範工具用法，例子可以直接在電腦上照住做，學完即刻用得到。
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1Computer images come in two families: bitmaps are built from a grid of pixels — photos are the classic case — while vectors record lines and shapes as mathematical formulas, common in logos and fonts. Enlarging a bitmap shows blocky squares (distortion), but a vector stays smooth at any size — the key difference.
2Resolution describes how fine an image is, counted in pixels: 1920 × 1080 means 1920 across and 1080 down. More pixels mean a sharper image and a bigger file. Phone photos are often 4000 × 3000 = 12 million pixels, which is why they take up real storage.
3Three basic edits: Crop trims away the surroundings to highlight the subject; Adjust changes brightness, contrast and saturation — a dark indoor photo brightened up; and Layers work like a stack of transparent sheets, with text and graphics on their own layers, so editing one layer leaves the others untouched.
4Choosing a save format matters too: JPG suits photos (small files, millions of colours), PNG supports transparent backgrounds (logos, stickers), and GIF makes simple animations. Think about the use before saving and you will never pick the wrong format.
1Summary 重點摘要
Bitmaps pixelate when enlarged, vectors stay smooth at any size; resolution counts pixels; crop, adjust and layer are the basic edits; JPG photos, PNG transparency, GIF animation.
2Vocabulary cards Vocabulary
An image made of pixel squares that pixelates when enlarged, e.g. phone photos.
An image recorded as formulas for lines and shapes, staying smooth at any zoom level.
The smallest dot of an image, one colour each; together they form the picture.
How fine an image is, in pixels, e.g. 1920 × 1080; higher is sharper but heavier.
Trimming the outer parts of an image to focus on the subject, without distorting it.
Adjustment controls: brightness sets overall lightness, contrast sets the dark-to-light gap.
An editing structure like transparent sheets; each element on its own layer, edited independently.
A lossy format with small files, ideal for photos; no transparency support.
A lossless format supporting transparency, ideal for logos and stickers.
3Step by step Steps
- Bitmap versus vectorComputer images fall into two families: bitmaps are grids of pixels — every photo in a phone album is one — while vectors record shapes as mathematical formulas and are used for school logos and fonts. The key difference appears when enlarging: a phone photo magnified eight times shows blocky mosaic squares, which is bitmap distortion, while a vector school logo printed on a giant banner keeps smooth edges. So watch the original resolution when editing photos, and ask designers for vector files for logos.
- Counting the pixelsPixel count equals width multiplied by height, and it determines both sharpness and file size. A 1920 × 1080 screen holds 1920 × 1080 = 2,073,600 pixels, about two million; a 4000 × 3000 phone photo has 12 million, so it is sharper but much larger, and filling a cloud album with such files consumes real storage space. Knowing this calculation explains why sending an original file costs several times the data of a compressed copy, and lets you estimate how much storage a batch of photos needs.
- CroppingCropping trims away the outer parts of a picture to highlight the subject. If an unrelated umbrella appears on the right of a group photo, frame the people with the crop tool and cut the edge away for a much cleaner picture; cropping removes only the outside and never distorts the subject. Remember that cropping reduces the pixel count, so heavy cropping followed by enlargement turns blocky — keep the original in the phone album, save the edited version as a new file, and check that a cropped post for social media is still sharp.
- Adjusting colourAdjusting changes brightness, colour temperature, contrast and saturation. Indoor photos often come out yellow and dark: raise the brightness and lower the colour temperature to restore natural skin tones. Drama or sports shots gain punch from extra contrast. Saturation makes colours vivid, but too much looks fake — purple skies and flushed faces. Phone album editors already offer these controls; adjust in small steps and compare with the original after every change.
- Thinking in layersLayers are like a stack of transparent sheets, one element per sheet, stacked into the full picture. A poster might use three layers: a background photo below, a semi-transparent colour block in the middle, and headline text on top; editing the text touches only the top layer, and moving the colour block never harms the photo. Layers let every element be revised independently and reordered freely; merging everything into one flat image means a single wording change may force a full remake.
- Resolution by purposeResolution should follow the purpose, and bigger is not automatically better. A social-media post needs only 1080 × 1080; a full A4 print needs 2480 × 3508 pixels (about 300 dpi) or the paper copy turns blocky; a wallpaper should match the screen's 1920 × 1080. Purpose decides resolution and file size together: uploading a 4000 × 3000 original to social media merely wastes data and time, while sending a small image to print produces blur that cannot be repaired.
- Choosing the formatThe save format decides a file's character and use. JPG is small and supports millions of colours, ideal for travel photos; PNG supports transparent backgrounds, ideal for school badges and stickers that sit on other artwork; GIF makes simple looping animations. Conversion loses information: saving a PNG as JPG turns its transparent areas white and they cannot be recovered. Before submitting homework or uploading, decide the use first — transparency needs PNG, ordinary photos take JPG, and animation uses GIF.
4Diagrams Diagrams
5Worked example Example
Example: a 1024 × 768 bitmap is doubled in both width and height. How many pixels does it then have, and what happens to the image?
- The original pixel count is 1024 × 768 = 786,432 pixels; note it down first as the baseline for comparison.
- Doubling both width and height gives the new size of 2048 × 1536.
- The new pixel count is 2048 × 1536 = 3,145,728, which is 4 times the original 786,432.
- Conclusion: doubling both sides multiplies the pixel count and the file size by four, and at the same time the image turns blocky, because a bitmap records the picture with a fixed number of pixels.
- Answer: The pixel count becomes 3,145,728 — four times the original — and the file is about four times larger; the enlarged image shows blocky distortion, because enlarging a bitmap only stretches each pixel and adds no detail.
6Common mistakes Mistakes
7Quick check Quick check
Q1The school logo must be enlarged onto a huge banner — bitmap or vector, and why?
Q2How many pixels does a 1920 × 1080 image contain?
Q3A photo is too dark — which editing tool fixes it?
Q4What is the advantage of keeping a poster's text and background on separate layers?
Q5Which format should a sticker needing a transparent background be saved as?
8Recap Recap
- Bitmaps are pixel grids that distort when enlarged; vectors never do.
- Resolution counts pixels — sharper means bigger files.
- Three basic edits: crop, adjust, layer.
- Layers are transparent sheets; edit one without touching the others.
- Formats: JPG for photos, PNG for transparency, GIF for simple animation.