Ubuntu Wiki

Library: uTouch-Physics
Language: C (Maybe built on top of GLib? Qt already depends on GLib by default if available.)
Dependencies: possibly a physics engine such as Chipmunk

Note: This page is in the process of being rewritten. It will be inconsistent until this task is finished.

User stories

The physics library should do following:

• provide a viewport that can be scrolled and which slows down according to physical laws
• provide several different behaviours when the area hits the edge: stopping, bouncing like Android's list widget etc
• support setting friction and other such variables
• provide (possibly system global) default values for the above
• allow changing the scrolling speed and direction even if there is a scroll event ongoing

Functionality it will not have:

• it does not convert coordinates, it will only work in pixel units
• it does not support on-the-fly area resizing because that behaviour is application dependent
• it is not thread-safe

Using the library

Note: the code samples in this section are descriptive. They may not line up exactly with the API described further in this document.

Creating a scroller

scroller = new_scroller(area_width, area_height, viewport_width, viewport_height);

Old content starts here.

Units

Screen coordinates make more sense than real-world coordinates. If you have a 30" monitor next to a 24" monitor, both with equal resolutions, scrolling should move the content the same amount in screen coordinates. To facilitate this, units of pixels are used for distance in internal physics calculations. When real-world units are used, the library will convert them to pixels internally before performing computation.

A 24" 1080p monitor is a typical size and resolution for a desktop computer. This works out to 3614 px/m in the diagonal direction. We will assume square pixels. Thus, a reasonable conversion from meters to pixels is about 3.5 MP/m (where MP is a mega-pixel, or 1000 pixels).

Models

Scrollview

This model may be used to facilitate calculations involving a scrollable view. The content is a rectangle, and the viewport is a rectangle whose area is a subset of the content. The content and viewport can be thought of as boxes. The viewport may be dragged within the content. The viewport cannot be dragged outside of the content.

While the user is interacting with the viewport by scrolling on an input device, the viewport follows the scroll motion. When the scrolling ceases, the viewport will continue moving with the velocity it has until friction or the content boundary force the viewport to a stop.

TODO: How can we add "bouncing" when the viewport hits the edge of the content area?

API Types

uphys

Handle for uTouch-Physics context.

uphys_model

Enum specifying the model of the uphys context.

Values:

• UPHYS_SCROLLVIEW

uphys_units

Enum specifying a unit type.

Values:

• UPHYS_MILLIMETER
• UPHYS_PIXEL

uphys_point

Holds a point in two axes.

uphys_size

Holds dimensions in two axes of an object.

uphys Properties

error: int

Error code from last function call.

Set to 0 on success.

error_string: const char *

Error string from last function call.

Set to "no error" on success.

gravity: float

Defaults to 34.3 MP/s². This is calculated by multiplying the standard earth gravity of 9.8 m/s² by the standard conversion from meters to pixels of 3.5 MP/m.

mm_multiplier: float

Scalar multiplier applied to parameters given in UPHYS_MILLIMETER units. Defaults to value read from dconf settings repository if available, and then to TBD.

This is roughly equivalent to the sensitivity of a trackpad.

mu: float

Friction constant. Defaults to value read from dconf settings repository if available, and then to TBD.

simulation_step_size: float

Simulation step size in ms. Defaults to TBD (maybe 30 Hz).

See here for more info.

(scrollview) viewport_origin: uphys_point

The origin point of the viewport of the scrollview.

The origin is defined as the point within the rectangle with the minimum values for both axes.

(scrollview) viewport_size: uphys_size

The size of the viewport of the scrollview.

(scrollview) content_origin: uphys_point

The origin point of the content of the scrollview.

The origin is defined as the point within the rectangle with the minimum values for both axes.

(scrollview) content_size: uphys_size

The size of the content of the scrollview.

Functions

int uphys_set_property(uphys *uphys, const char *name, void *value)

Set a property value.

Returns 0 if successful, non-zero on error.

int uphys_get_property(const uphys *uphys, const char *name, void *value)

Get a property value.

Returns 0 if successful, non-zero on error.

void uphys_inject(uphys *uphys, GeisEvent event)

Inject an input event.

An error will occur if the time of the event is earlier than the time of a previous uphys_update() call.

void uphys_update(uphys *uphys, time_t *time)

Perform simulation of model up to the time given.

uphys *uphys_new(uphys_model model)

Create a new uphys context for a the model.

Returns a new context or NULL on error.

API Use Case Example

Implementing smooth scrolling in a document viewer.

Setup steps:

1. Create a uphys context using uphys_new(UPHYS_SCROLLVIEW)
2. Set properties on the context for the origin and size of the content and viewport
3. Create a Geis context to receive drag events
4. Add a callback for uphys on the display refresh signal

   • - Use OpenGL sync signal if available
     - Otherwise use a static timer at a reasonable interval (maybe 30 Hz)

In Geis drag callbacks:

1. Call uphys_inject().

   • - For touchpads and independent devices use UPHYS_MILLIMETER units
     - For touchscreens use UPHYS_PIXEL units

In the uphys callback:

1. Call uphys_update() with the current time
2. Get the new viewport origin using uphys_get_property()
3. Redraw the scrollview