Lessons 6 · 1 lessons · N. I. Taylaqov (general editor), A. B. Akhmedov, M. D. Pardayeva, A. A. Abdug‘aniyev, U. M. Mirsanov. Informatics and Information Technologies, Grade 11, 1st edition. “Extremum-press”, Tashkent, 2018
6
Selecting a geometric-shaped part of an image in PhotoShop
Textbook: pp. 23–27
GoalSelects an area with the selection tools (rectangle, ellipse, single row/column, lasso), adds to, subtracts from and intersects selections, and cancels a selection.
New words
selection: a moving dashed outline around the part of a picture that will be changed · ajratmaMarquee tools: select a rectangular, elliptical, single-row or single-column area · Marquee uskunalariLasso: a tool that selects a free-form area you draw by dragging the mouse · Lasso (Arqon)intersection: the part that belongs to both the old and the new selection · kesishma
Explanation
In text or a vector drawing it is easy to pick a part, because they consist of a few characters or objects. A raster has millions of pixels that cannot be picked one by one, so the boundary of the part is marked; the selected part is surrounded by a moving dashed line and only that part changes. Ctrl+A selects the whole image and Ctrl+D (Select → Deselect) cancels a selection. The first group on the toolbar is Marquee: rectangular and elliptical area, single row and single column tools (key M); you press the left button, drag along a diagonal and release. The options bar has four modes: new selection, add (Shift), subtract (Alt) and intersect (Shift+Alt); when a new area is selected, the old selection is replaced, added to, subtracted from or intersected according to the mode. For complex shapes the Lasso (key L) and Polygonal Lasso are used: with the first you draw the boundary along the mouse trail, with the second you click the ends of straight segments. A selected part can be copied with Ctrl+C and pasted with Ctrl+V.
Worked examples
A round target: with the Elliptical Marquee select a 200×200 pixel ellipse, then hold Alt and subtract a 100×100 pixel ellipse from the inside. The result is a ring, which can be filled with colour using Edit → Fill (Shift+F5). Ctrl+D cancels the selection.
Counting pixels: rectangle A is 150×80, rectangle B is 100×80, and they overlap in a 60×80 pixel part. Added together: 150·80 + 100·80 – 60·80 = 15 200 pixels; B subtracted from A: 150·80 – 60·80 = 7 200; the intersection is 60·80 = 4 800 pixels.
Class activity
“Selection workshop”: pairs use the selection modes to paint a “house” silhouette (a square plus a triangular roof); a neighbouring pair works out which modes were used.
Practice
1
Name the four tools of the Marquee group.
Rectangular marquee, elliptical marquee, single row, single column.
2
Rectangle A is 200×100 pixels, rectangle B is 100×100 pixels, and they overlap in a 50×100 pixel part. How many pixels are in their union?
25000
3
Which key adds to a selection, which subtracts, and which intersects?