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Jeffrey Ratcliffe > Gtk2-ImageView > ImageToolSelector.xs


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Gtk2::ImageView::Tool::Selector is a tool for selecting areas of an image. It is useful for cropping an image, for example. The tool is an implementor of the Gtk2::ImageView::Tool inteface which means that it can be plugged into a Gtk2::ImageView by using the Gtk2::ImageView::Tool::set_tool() method.

Gtk2::ImageView::Tool::Selector changes the default display of the Gtk2::ImageView. It darkens down the unselected region of the image which provides a nice effect and makes it clearer what part of the image that is currently selected. Unfortunately, this effect is somewhat incompatible with how Gtk2::ImageView::Nav behaves because that widget will show the image without darkening it.

The tool also changes the default behaviour of the mouse. When a Gtk2::ImageView::Tool::Selector is set on a Gtk2::ImageView, mouse presses do not "grab" the image and you cannot scroll by dragging. Instead mouse presses and dragging is used to resize and move the selection rectangle. When the mouse drags the selection rectangle to the border of the widget, the view autoscrolls which is a convenient way for a user to position the selection.

Please note that Gtk2::ImageView::Tool::Selector draws the image in two layers. One darkened and the selection rectangle in normal luminosity. Because it uses two draw operations instead one one like Gtk2::ImageView::Tool::Dragger does, it is significantly slower than that tool. Therefore, it makes sense for a user of this library to set the interpolation to GDK_INTERP_NEAREST when using this tool to ensure that performance is acceptable to the users of the program.

Zoom bug

There is a small bug in Gtk2::ImageView::Tool::Selector that becomes apparent when the zoom factor is greater than about 30. The edge of the selection rectangle may in that case intersect a pixel.

The bug is caused by bug 389832 in gdk-pixbuf. There is no way to solve this bug on Gtk2::ImageView's level (but if someone knows how, I'd really like to know).

selection : (0, 0) - [0, 0]

This method does nothing under the following circumstances:

If the views pixbuf is undef.
If rect is wider or taller than the size of the pixbuf
If rect equals the current selection rectangle.

If the selection falls outside the pixbufs area, its position is moved so that it is within the pixbuf.

Calling this method causes the ::selection-changed signal to be emitted.

The default selection is (0,0) - [0,0].

selector : a Gtk2::ImageView::Tool::Selector
rect : Selection rectangle in image space coordinates.
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