Using rectangle shape is the most versatile instrument for selection, indeed, you start with a rectangle and then reshape it into various quadrilaterals, such as square, parallelograms, trapezoids, and rhombi. Moreover, app allows you to reshape rectangle even to triangle.
Working with rectangle
Start from selecting color you like
and then select rectangle shape.
There are four grippers in each corner of rectangle to resize and reshape it
You can move selection using mouse - simply click and hold your mouse inside the selection area and drag it to the desired location. Don't worry about perfect shapes: focus on selecting the specific part of the image you want, and refine the selection later if needed..
Use
to show and hide grid.
Right-click (or long-press) on the gripper to access a menu: "Snap to grid" menu item aligns the active gripper
to the nearest grid point, "Make rectangle" menu item straightens all angles of the selection while keeping the
active gripper in place. This creates a perfect rectangle.
Overlap two adjacent grippers to attchive triangle shape. Needless to say, all manipulations with
rectangle work for triangle as well - you can adjust the triangle's size and position just like a
rectangle using the grippers.
When you are satisfied with selection use
to open new image-editing window and copy selection to it. In the upper-left
corner of this new window find the save button
to save your work.