Vortex AGI Tutorials - Visual vs Priority

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Visual vs. Priority Backgrounds
By nemesis


You probably recall that earlier I mentioned that AGI picture resources include two very important parts, the visual background and the priority background. So far, we have been working with the visual background. However, in order for your game to look 3D, you must have a priority background as well. The priority background consists of two parts: control lines and priority bands.

Control Lines

Control lines are used to tell the computer where characters are able to walk and where there are obstacles, such as walls, furniture, trees, or whatever else might prevent characters from being in a certain area.

On the priority screen, the first four colors (0 - 3) have special meanings.

  • 0 (black) - Unconditional barrier. No object is ever allowed to be on parts of the screen with a pixel of this color in the priority screen.
  • 1 (dark blue) - Conditional barrier. Objects are not normally allowed to be on a conditional barrier, unless the ignore.blocks command is used to allow them to do so. (more on the ignore.blocks commands in the logic section)
  • 2 (dark green) - Signal barrier. This can be used to test if a character is walking in a certain place. Particularly useful for doors, or edges of cliffs, etc. Wherever something will happen when a character walks over that place.
  • 3 (dark cyan) - Signifies water. Parts of the priority screen which are not color 3 are considered to be land. Color 3 can also be used to represent some other surface such as ice, lava, sand, a slippery floor, or whatever you need for that particular room.

Using these four colors, you should be able to effectively control where a character or object can be.


Priority Bands

The AGI screen is divided vertically into 12 priority bands (colors 4 - 15). Objects with a higher priority are displayed on top of objects with a lower priority. Different parts of a picture can also be given different priorities. So something in the far distance would have a priority of 4 (everything else would go on top of this) and something right at the front of the screen would have a priority of 15 (nothing will go on top of this except objects with a priority of 15).

In PicEdit, load picture.16. Press the TAB key to switch into priority mode. The visual background will disappear, and the priority background will be visible instead. The control lines are easily recognizable and the priority bands show up as colored shaded areas. Notice the value next to Pri: in the status bar at the top of the editor screen. This number represents the priority band that corresponds with the current Y-position. The best way to understand priority backgrounds is to look at several AGI games' picture resources and study their priority screens.

Control lines and priority bands are drawn in the same way as the visual background. In fact, you can draw your priority background at the same time you draw the visual. Simply right click on the priority color that you want to use before you draw the visual picture. Remember to click on the off button with the right mouse button when you want to work on the visual screen only, and with the left mouse button when you want to draw on the priority screen only. Hint: you can use the navigation buttons to go back and add priority bands and control lines as their visual counterparts are drawn.


Exercise 1.4 Priorities

Open PicEdit with Template.pcx as a background, then open the picture that you saved in the previous exercise. Using the template as a guide for your priority bands, draw the priority screen for your picture. Remember to save your work.


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