Description:
Assumptions about users/diagrams:
1. Most people have their attention on the center of the page, and the center of the page represents their current area of concern or focus.
2. People want to keep their area of focus the same while zooming, and don't want interruptions due to manual repositioning of the page.
3. To increase usability, users try to layout tables in a visual space with a view to keeping related tables as near to each other as is reasonable.
4. Fan-out occurs, and central tables have a tendency to have a cluster of related tables near them.
Description of current behaviour:
When changing the zoom on the ERD, the page zooms relative to the top left corner of the window.
Reason against current behaviour:
As zoom increases, the information focused on the centre of the page drifts off the bottom right, the user must tracks it visually until it is off the page. The focus of attention is lost, then the user must repositon the window page using the cursors or the reposition hand, this extra step is inefficient.
As zoom decreases, the information on the centre of the page drifts into the top left, the user tracks it visually. If it is the intention of the user to look at related tables surrounding the object table, then the user only gets the benefit of relationships to the bottom right, however, any tables to the top left would require repositioning of the window page. Functionally, it is ineffective for looking at all fan-out relationships that may be top left of the area of focus.
I suggest that there would be less steps, less interruptions, more efficiency and therefor more functional if zooming occurred from the middle of the window where the user focus is, because the user will have a reduced need to reposition the window using the cursor or the hand.
How to repeat:
Layout an ERD.
Focus on a table in the center screen with a large fan-out of relations.
Change the zoom in and out, trying to keep visual reference to the other tables.
Note how top-left and bottom-right motions reduce your ability to track information.
Use the slider for a better visual of the behaviour.
The scaling is relative to the top left of the page.
Suggested fix:
Scale from the centre of the page.
The resulting zoom will make it possible for the user's focus to stay on the area he/she is currently focused on.
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