What is Evas?¶
Evas is a clean display canvas API for several target display systems that can draw anti-aliased text, smooth super and sub-sampled scaled images, alpha-blend objects and much more.
It abstracts any need to know much about what the characteristics of your display system are or what graphics calls are used to draw them and how. It deals on an object level where all you do is create and manipulate objects in a canvas, set their properties, and the rest is done for you.
Evas optimises the rendering pipeline to minimise effort in redrawing changes made to the canvas and so takes this work out of the programmers hand, saving a lot of time and energy.
It’s small and lean, designed to work on embedded systems all the way to large and powerful multi-cpu workstations. It can be compiled to only have the features you need for your target platform if you so wish, thus keeping it small and lean. It has several display back-ends, letting it display on several display systems, making it portable for cross-device and cross-platform development.
What Evas is not?¶
It is not dependent or aware of main loops, input or output systems. Input should be polled from various sources and fed to Evas. Similarly, it will not create windows or report windows updates to your system, rather just drawing the pixels and reporting to the user the areas that were changed.
How does Evas work?¶
Evas is a canvas display library. This is markedly different from most display and windowing systems as a canvas is structural and is also a state engine, whereas most display and windowing systems are immediate mode display targets. Evas handles the logic between a structural display via its state engine, and controls the target windowing system in order to produce rendered results of the current canvas’ state on the display.
Immediate mode display systems retain very little, or no state. A program will execute a series of commands, as in the pseudo code:
draw line from position (0, 0) to position (100, 200); draw rectangle from position (10, 30) to position (50, 500); bitmap_handle = create_bitmap(); scale bitmap_handle to size 100 x 100; draw image bitmap_handle at position (10, 30);
The series of commands is executed by the windowing system and the results are displayed on the screen (normally). Once the commands are executed the display system has little or no idea of how to reproduce this image again, and so has to be instructed by the application how to redraw sections of the screen whenever needed. Each successive command will be executed as instructed by the application and either emulated by software or sent to the graphics hardware on the device to be performed.
The advantage of such a system is that it is simple, and gives a program tight control over how something looks and is drawn. Given the increasing complexity of displays and demands by users to have better looking interfaces, more and more work is needing to be done at this level by the internals of widget sets, custom display widgets and other programs. This means more and more logic and display rendering code needs to be written time and time again, each time the application needs to figure out how to minimise redraws so that display is fast and interactive, and keep track of redraw logic. The power comes at a high-price, lots of extra code and work. Programmers not very familiar with graphics programming will often make mistakes at this level and produce code that is sub optimal. Those familiar with this kind of programming will simply get bored by writing the same code again and again.
For example, if in the above scene, the windowing system requires the application to redraw the area from 0, 0 to 50, 50 (also referred as “expose event”), then the programmer must calculate manually the updates and repaint it again:
Redraw from position (0, 0) to position (50, 50): // what was in area (0, 0, 50, 50)? // 1. intersection part of line (0, 0) to (100, 200)? draw line from position (0, 0) to position (25, 50); // 2. intersection part of rectangle (10, 30) to (50, 500)? draw rectangle from position (10, 30) to position (50, 50) // 3. intersection part of image at (10, 30), size 100 x 100? bitmap_subimage = subregion from position (0, 0) to position (40, 20) draw image bitmap_subimage at position (10, 30);
The clever reader might have noticed that, if all elements in the above scene are opaque, then the system is doing useless paints: part of the line is behind the rectangle, and part of the rectangle is behind the image. These useless paints tend to be very costly, as pixels tend to be 4 bytes in size, thus an overlapping region of 100 x 100 pixels is around 40000 useless writes! The developer could write code to calculate the overlapping areas and avoid painting then, but then it should be mixed with the “expose event” handling mentioned above and quickly one realizes the initially simpler method became really complex.
Evas is a structural system in which the programmer creates and manages display objects and their properties, and as a result of this higher level state management, the canvas is able to redraw the set of objects when needed to represent the current state of the canvas.
For example, the pseudo code:
line_handle = create_line(); set line_handle from position (0, 0) to position (100, 200); show line_handle; rectangle_handle = create_rectangle(); move rectangle_handle to position (10, 30); resize rectangle_handle to size 40 x 470; show rectangle_handle; bitmap_handle = create_bitmap(); scale bitmap_handle to size 100 x 100; move bitmap_handle to position (10, 30); show bitmap_handle; render scene;
This may look longer, but when the display needs to be refreshed or updated, the programmer only moves, resizes, shows, hides etc. the objects that need to change. The programmer simply thinks at the object logic level, and the canvas software does the rest of the work for them, figuring out what actually changed in the canvas since it was last drawn, how to most efficiently redraw the canvas and its contents to reflect the current state, and then it can go off and do the actual drawing of the canvas.
This lets the programmer think in a more natural way when dealing with a display, and saves time and effort of working out how to load and display images, render given the current display system etc. Since Evas also is portable across different display systems, this also gives the programmer the ability to have their code ported and displayed on different display systems with very little work.
Evas can be seen as a display system that stands somewhere between a widget set and an immediate mode display system. It retains basic display logic, but does very little high-level logic such as scrollbars, sliders, push buttons etc.
Object may carry hints, so that another object that acts as a
manager may know how to properly position and resize its subordinate objects.
The Size Hints provide a common interface that is recommended as the protocol for such information.
For example, box objects use alignment hints to align its lines/columns inside its container, padding hints to set the padding between each individual child, etc.
Size Hints are controlled using various
The weight and the align are quite special, they are also used to express the expand and the fill property of the object. For this reason some helper are provided:
EVAS_HINT_EXPAND= -1.0 (to be used with weight or expand)
EVAS_HINT_FILL= 1.0 (to be used with align or fill)
EXPAND_BOTH= EVAS_HINT_EXPAND, EVAS_HINT_EXPAND
EXPAND_HORIZ= EVAS_HINT_EXPAND, 0.0
EXPAND_VERT= 0.0, EVAS_HINT_EXPAND
FILL_BOTH= EVAS_HINT_FILL, EVAS_HINT_FILL
FILL_HORIZ= EVAS_HINT_FILL, 0.5
FILL_VERT= 0.5, EVAS_HINT_FILL
You can also build your own as needed, for example you can define:
FILL_AND_ALIGN_TOP= EVAS_HINT_FILL, 0.0
FILL_AND_ALIGN_RIGHT= 1.0, EVAS_HINT_FILL
and so on…
After you understood what Evas is and installed it in your system you should proceed understanding the programming interface for all objects, then see the specific for the most used elements. We’d recommend you to take a while to learn Elementary and Edje as they will likely save you tons of work compared to using just Evas directly.