The Banner Window Display Model

The tads-io function set provides a group of functions that allow the program to perform sophisticated manipulation of the display layout. This group of functions, called the Banner Window API, lets the program divide the screen into separate "window" areas that can show independent information.

To use the Banner API, it's necessary to understand its underlying display model. This section explains the display model. The API functions are part of the tads-io function set, so refer to that section for details on the functions.

Screen layout overview

Banners work by splitting one window into two. In other words, we take an existing window and draw an imaginary line either across the width of the window, or down the height of the window; the existing window keeps the space on one side of the line, and the new banner window gets the space on the other side of the line. The existing window shrinks in the process, since some of its space is taken over for the new banner window.

The original window that we split in two is called the "parent" window. The parent can be the main game window (the original window that the interpreter automatically creates at start-up, and which contains the main input/output transcript of the game), or it can be another banner window. We sometimes refer to the new banner window as a "child" of the parent window.

When the program creates a banner window, it specifies an "alignment" and a size for the new window. The alignment determines how we draw the line that splits the parent window, and which side of the line contains the banner window and which contains the shrunken parent window. There are four types of alignment: Top, Bottom, Left, and Right. For Top and Bottom alignment, we split the window with a line across the width of the window; for Top alignment, the new banner window gets the space above the line, and the for Bottom alignment, the new banner takes the space below the line. Left and Right alignment split the window with a vertical line; a Left banner takes the portion to the left of the line, and a Right banner takes the portion to the right of the line. The size determines where the dividing line goes: the dividing line is placed such that the banner's height (for a Top or Bottom banner) or width (for a Left or Right banner) matches the size parameter.

To determine where each banner goes, the interpreter does the following. We start with the main window, giving it the entire available space (this might be the entire screen on a character-mode terminal, or the entire application window on a graphical system). We then visit each child of the main window. For each child, we look at the child's alignment and size to determine where to draw the imaginary dividing line; then we give the banner window the appropriate portion, and shrink the main game window to take its portion of the space. Then, we visit all of the child's children, and do the same thing, dividing and shrinking the child. We repeat the process on the child's children's children, and so on until we run out of descendants.

Let's look at an example. Initially, we start out with just the main game window, which takes the entire available space:


Now, let's add a banner. Text games traditionally display a status line at the top of the screen, with one or two lines of text; so, let's create a banner for the status line. For now, we'll make it one "text unit" high, which is roughly a line of text. So, we're creating a banner whose parent is the main window, with Top alignment and a size of 1.


Now let's suppose that we want to add a more complex display, with an area below the status line that displays a picture of the current location. We want another top-aligned child of the main window, because we want it to run the width of the main window and be above the main window. We'll make its size about ten lines for now.


This raises a question: why is the Picture window below the Status window? And what if we actually wanted it above the Status window? The answer is in the ordering of the windows: the Status window comes first in the main window's child list, so it gets split off first, which means it gets the top of the entire display area. The Picture window is split off second, which means that it has to take what's left of the main window after the Status window has already been carved out. If we wanted the Picture window to be above the Status window, we'd simply have to specify that it comes first. Note that this doesn't mean we have to create it first; when we create a new banner, we can specify where the new banner goes among the other children of its parent, so if we had wanted the Picture window to be on top, we could simply have specified that it goes first in the main window's child list.

Now we want to add another window: we want to add a window alongside the picture window, showing a compass graphic in the new window. We don't want to create another "band" across the entire width of the main window; rather, we want to share some of the space the Picture window already has. To do this, we can create a child of the Picture window, and align it to the right.


The Compass ("C") window splits the area that the Picture window had, taking the right portion because it's a right-aligned banner.


A banner can optionally display a "border" to separate it from its parent. The border essentially makes the imaginary dividing line between the banner and its parent visible on the screen. Borders are controlled via the "style" flag when creating a banner.

A banner's border is always exactly one line: it's always the dividing line between the banner and its parent. In the example above with the Picture and Status banners, if the Picture banner had a border, it would appear between the Picture and Main windows, because the Picture window was split from the Main window. To draw a line between the Status and Picture windows, you would have to give the Status banner a border.

A banner's children have no effect on the banner's border. If a banner is drawn with a border, the border will appear exactly the same way whether the banner has children or not; so, to figure out where a border would be drawn, consider what the window would look like without any children of its own. In the example above with the Picture and Compass windows, if the Picture window had a border, this border would be drawn across the full width of the window, under both the Picture and Compass ("C") windows. So, there would be a border between the Compass window and the Main window whether or not the Compass window itself had a border. (Furthermore, if the Compass window did have a border, it would be drawn as a vertical line between the Compass and Picture windows, because the Picture window is the Compass window's parent.)

Some systems do not support borders at all; in fact, most character-mode platforms do not, because there's usually no good way to draw lines in character mode, and even if there were, it would take up too much space. You can tell whether or not the current system supports border drawing by checking the style flag returned from bannerGetInfo(); on systems that don't support borders, you might want to use some other means to visually separate a banner from its parent, such as using different background ("screen") colors in each window.

More Details on Layout

TADS interpreters display everything on what we call "the screen". This term is misleading on modern systems, because most computers today have graphical user interfaces, and on a GUI, TADS usually only gets a window on the desktop, not the whole physical display. To clear up any confusion, here's what "the screen" actually means to TADS in the most common configurations:

When the TADS interpreter loads a new game program, it initially devotes the entire "screen" to one window, which we call the "main game window". This main window acts a lot like a banner window, but it has some differences, so we use the special designation "main game window" to refer to this window.

At any time while the game program is running, it can create new "banner windows". A banner window is a rectangular area of the screen that can be separately controlled: each banner has its own contents, background color, text color, and so on.

All of the banner windows that are shown at any given time share space on the screen with one another and with the main game window. Two windows can never overlap, every window is rectangular, and all of the windows taken together must fill the entire screen. In other words, the windows act as non-overlapping "tiles" that completely fill the available screen space. These are important constraints that determine precisely how the screen looks with a given arrangement of windows, and make it relatively simple to specify the screen layout; the disadvantage of these constraints is that they don't allow the game program to create any arbitrary display, but the compensating advantage is that the constraints are simple to apply across a wide range of platforms, so the program can count on consistent results without having to include special-case code for different machines and display types.

Each banner window has three main parameters that control its layout on the screen. First, each banner has a "parent": this is either the main game window or another banner window, and provides the base area from which the new banner's area is taken. Second, each banner has an alignment type, which can be Top, Bottom, Left, or Right. The alignment specifies which portion of the parent window's space is taken for the banner. A top-aligned banner takes the top portion of the parent's space, and runs the full width of the parent window; a bottom-aligned banner also runs the entire width of the parent, but takes the bottom portion of its space. Left and right banners run the entire height of the parent window, and take the left and right side, respectively, of the parent's space. Third, each banner has a size, which gives the size of the "free" dimension, as determined by the alignment: a top or bottom banner's size specifies its height, and the left or right banner's size specifies its width.

Each parent window can have one or more children. When a window has more than one child, the children are arranged into an ordered list: one banner is the first child, another is the second, and so on. The order is important because each child is carved out of its parent's remaining space; we start with the first child, taking its space away from the parent's space, then move to the second child, taking its space away from what remains of the parent's space, and so on. When the game program creates a banner, it can specify the position of the new banner among the parent's children.

The screen layout is dynamic: each time a new banner window is created, or a banner window is removed, or a banner is resized, or the screen size changes (which can happen, for example, when the user resizes the "xterm" window displaying the interpreter), TADS must recalculate the screen layout. Every time any of these events occurs, TADS completely recalculates the screen layout; this means that the layout is always predictable, because it's always calculated the same way, regardless of what caused a change.

To calculate the screen layout, the interpreter first gives the main game window the entire screen area. The interpreter then visits each child of the main game window, in the order that the program specified when creating the child windows. For each child window, we do the following:

  1. We look at the current child window's alignment and size settings, and figure out where to draw the imaginary line dividing the parent window. If the child's size setting is greater than the available space in the parent window, we limit the child's size to that of the parent window; in other words, the child gets all of the parent's space (but no more), and the parent will get nothing.
  2. We assign the space on one side of the dividing line (according to the alignment) to the current child.
  3. We assign the space on the other side of the line to the parent, shrinking the parent to its new size.
  4. For each child of the current child window, we recursively apply this same procedure to subdivide the current child's space among its own children.

There are a couple of interesting features of this layout algorithm worth pointing out.

First, when a banner is deleted, all of its children become immediately invisible. This is because a banner gets its space exclusively from its parent; once a banner is deleted, its children no longer have any way to obtain any space on the display. It won't even help to resize the children: without a parent, a child simply has no way to obtain any display space. The children of a deleted banner remain valid as abstract objects, so you can still call banner API functions that reference their banner handles, but they can never again have a display presence.

Second, a banner's final size is not necessarily the same as its size parameter setting. Suppose we have a top-aligned banner (call it "A"), a child of the main game window with a size of 50%. Suppose also that this window has a child of its own (call this "B"): a bottom-aligned banner which also has a size of 50%. The layout algorithm will start by giving banner A the top half of the entire screen; but when we lay out its child B, we take away half of that height to give to B. In the end, A only gets 25% of the entire screen. (This probably won't come up a lot in practice, since it would be unusual to create a child with the same orientation as its parent.)

Third, a child window can never cause its parent to expand. If a child's size is greater than the available space in its parent, the child simply gets the entire available space in the parent - but no more - and the parent is shrunken down to zero size.

Fourth, although a child causes its parent to shrink as the child carves out space from the parent, this effect never flows "up the tree": a child can never affect its grandparent or any siblings of its parent. This is an important but subtle point, because it makes the layout rules simple and predictable; there is no "negotiation" for space, and no possibility of irresolvable conflicts, as can sometimes occur with constraint-based layout systems.

Banner Types

When the game creates a banner window, it can specify the type of window to use. The following types are defined:

Note that text grids are mostly provided for character-mode platforms, but they're also supported on the full HTML interpreters for compatibility. In most cases, the kinds of effects for which text grids are most useful can be presented more elegantly on the full HTML interpreters by other means; for example, a "menu" window that uses cursor positioning to move around a selection pointer in response to keyboard inputs can often be better presented in HTML interpreters using a list of hyperlinks. Whenever you're about to use a text grid, you should at least consider whether there might be another alternative that's a better fit for GUI platforms.

Sizing Banners

The banner API provides three different ways of setting the size of a banner. Which sizing method is best for a particular banner depends on how the banner is used.

Note that when we talk about the "size" of a banner, we're talking about the free dimension of the banner, because the other dimension is always bound by the layout rules and thus can't be controlled independently. For a top-aligned or bottom-aligned banner, the size is the height; for a left- or right-aligned banner, the size is the width.

Percentage sizing: The first method is to size a banner in proportion to the available parent area, by specifying the size as a percentage when creating the banner (with bannerCreate()) or changing its size (with bannerSetSize()). The size of the banner will always be the given percentage of the available parent space; see the screen layout overview above for details of how this is determined. When you set a percentage size, the banner specifically remembers the percentage value, not the pixel size that the percentage translates into at any given time; whenever the layout changes (because the user resizes the application window, for example, or because another banner is created), the banner automatically adjusts to the new layout using the same percentage.

Absolute sizing: The second method of sizing a banner is to state the size in terms of the "natural units" of the banner window. The natural units depend on the type of banner:

Content-based sizing: The third method is to size the banner so that its current contents exactly fit. In many cases, it's desirable for a banner to be exactly big enough to show some specific contents; this is the case when a banner is used to display something like a status line or an interactive menu. Even absolute sizing is inherently imprecise for regular text windows, because the natural sizing units are character cells that might not represent the actual mix of characters and fonts displayed. To accommodate the need for exact content-based sizing, the banner API provides a function, bannerSizeToContents(), that sets the size of a banner window so that the actual current contents exactly fit.

By default, the content size of a banner includes only the banner's own contents. If the banner has any child banners, they're not normally included in calculating the banner's content size. However, sometimes it's useful to be able to include some or all of the child banners in calculating the size of the parent. For this, you can use the BannerStyleHStrut and BannerStyleVStrut style flags in the child banners whose sizes you want to include. ("Strut" in the sense of a rigid support bar: the child window's contents serve to force the parent window to at least the size of the child.) If you set the BannerStyleHStrut style on a child window, bannerSizeToContents() takes the child's width into account when setting the parent's width. Likewise, if you set BannerStyleVStrut on a child, the child's height will be figured into the parent's height.

Unfortunately, not all platforms are technically capable of supporting contents-based sizing, so it's not available on all interpreters. It's always safe to call bannerSizeToContents(), but on platforms where contents-based sizing isn't supported, the function will simply do nothing. Because of this, you should always be careful when using bannerSizeToContents() to also set an "advisory" size. Platforms that support true contents-based sizing will ignore the advisory and use the real content size, and those that don't support contents-based sizing will simply use the advisory size. To make sure your banners work properly on different platforms, follow this recipe whenever you use contents-based sizing:

  1. Calculate an estimate of the required size, in the window's natural units. For text windows, this is usually just a matter of determining how many lines of text will be displayed. Of course, it's not always possible to predict the exact number of lines that you'll need, especially if line wrapping comes into play. If you're displaying text with lines long enough that they might wrap some of the time, you might want to include a couple of lines of padding in your estimated size to be safe.
  2. Call bannerSetSize() to set the banner to the size you estimated. Pass true for the isAdvisory flag, to indicate that an exact contents-based size will be set later. Platforms that support contents-based sizing will ignore this call entirely, because of the isAdvisory flag, so there will be no unnecessary redrawing from an extra resize.
  3. Display the contents.
  4. Call bannerSizeToContents() to set the exact contents-based size, if possible. For platforms where this is not implemented, it will have no effect, so the estimated size will remain in effect; where contents-based sizing is available, the estimated size will be discarded and the exact size set instead.

By using this procedure, you'll ensure that your banners will look reasonably good on all platforms, and that it they'll look exactly right on platforms that do support size-to-contents (as most platforms do).

Note: in practice, you'll probably never encounter a platform where bannerSizeToContents() doesn't work. The recipe above was designed in anticipation of Glk-based implementations of HTML TADS and the banner API. (Glk is a UI portability toolkit for IF-style user interfaces. Glk doesn't provide a means of calculating exact layout sizes, so bannerSizeToContents() could not have been implemented in Glk versions.) As it turned out, no one ever built Glk ports with the banner API. Instead, full native versions are available on all of the major desktop platforms, and all of these support the more reliable bannerSizeToContents() functionality.

Banner Functions

The banner API is implemented as part of the tads-io function set, which provides programmatic control over the TADS user interface. The banner functions all have names of the form bannerXxx.

Notes for TADS 2 users

The banner API replaces the TADS 2 <BANNER> tag. The <BANNER> tag is not supported at all in TADS 3; interpreters will ignore it.

The Banner API provides the same functionality that was available in HTML TADS 2 interpreters via the <BANNER> tag, but with many improvements. The new API goes beyond the <BANNER> tag from TADS 2 in several important ways: