Blitter Objects

In this Chapter you will learn how to take full advantage of the Amiga's "Blitter" chip, which can copy large sections of a screen almost instantaneously.

At its fastest, the Blitter can move a million screen points per second, which is the equivalent of a dozen graphic screens. AMOS Professional exploits this facility for the incredible speed achieved in commands like SCREEN COPY, but the Blitter is capable of far more than simple graphics.

Professional animations are readily available, using special "Blitter Objects" known as "Bobs". Bobs can be displayed at any point on the screen and freely moved over the entire screen area, without disturbing any existing graphics. They may be guided, tested for collisions and even animated with AMAL, exactly like Sprites.

The main advantage of Bobs over Sprites is that they are far easier to use. There is no imit to the size or number of Bobs, and they are stored as part of the current screen, so all positions are measured in simple screen coordinates. As has been explained in the previous Chapter, Sprites only work in certain combinations, but Bobs may be displayed with no restrictions at all, at any position, and in vast numbers. The only limit is the amount of available memory! The other main advantage over Sprites is that Bobs can have up to 64 colours.

Naturally, all this power carries a price tag, and although Bobs are more flexible than Sprites, they are also slightly slower and consume additional memory. So the ideal solution is to use both Sprites and Bobs to their full advantage in the same program. They make a superb team, just like your Amiga, AMOS Professional and you!

Displaying a Bob

Images to be used as Bobs are stored in memory Bank 1, and are each referred to by a simple number, which ranges from 1 up to the maximum number of objects in the bank. Load up some images now, like this:

To find how many objects are in memory bank 1, use the LENGTH function for an instant read- out. Type this next line from Direct Mode:

This Object Bank is also used for any Sprite images, so the same objects can be displayed as Bobs or Sprites with great ease. To create a Bob, the image of an object is taken from the bank, and allocated for display as follows.

BOB

Each Bob must be given an identification number from 0 to 63. As a default, only 64 Bobs may be displayed on screen at once, but this limit can be increased if necessary.

Unlike Sprites, which use complex hardware coordinates, Bobs are displayed using standard screen coordinates, measured from the top left-hand corner of the current screen. Set the position of your new Bob by giving it screen coordinates relative to the hot spot of your chosen image number. Hot spots are explained at the end of the last Chapter.

If the coordinates lie outside of the existing screen area, the Bob will not be displayed. So objects can be initialised off screen, ready to be moved into place during the course of your program.

Once a Bob has been positioned on screen, the coordinate values become optional. The values of any coordinate parameters that are omitted will be remembered from the last time they were set. In Chapter 7.6 it is explained how this technique is valuable for animating Bobs with AMAL, because it allows objects to be moved effortlessly, without disturbing any existing animation sequences. It is vital to include all commas in their normal positions if coordinate values are omitted, or a syntax error will be reported. For example:

Before examining the next instant demonstration program, here is a step-by-step technique for correctly displaying a Bob.

• First, some images must be made available for Bobs to use, with a call to LOAD the appropriate filename. Once images have been loaded, they are saved as part of your Basic program automatically.
• If you intend to load a picture for use as a background screen, now is the time to do it. Use a line such as:

Alternatively, the default screen can be prepared by removing the flashing cursor from the display and filling the display with a large block of colour, usually black. For example:

• Now the correct image colours should be grabbed from the Object Bank. Note that if Bobs are to be displayed against an existing background screen, you will need to ensure that the images use exactly the same colour values as your picture, otherwise serious colour clashes will be generated. GET BOB PALETTE can be called if you are using Bobs on their own, or call GET SPRITE PALETTE for use with either Sprites or Bobs.
• The automatic AMOS Professional "double buffering" system should now be engaged, with a simple DOUBLE BUFFER command. The theory and practice of this is explained later, but in

essence, double buffering creates an invisible copy of the current screen where drawing operations take place, resulting in beautifully smooth movement effects.
• Finally, your Bobs are assigned their individual starting positions. This could be a simple series of BOB commands, or a complex pattern of off-screen starting points for each level of an arcade game.

General Bob Commands

BOB OFF

instruction: remove a Bob from display
Bob Off
Bob Off number

Use this command to remove all Bobs from the screen simultaneously. If a Bob number is specified, only that Bob will be extinguished. For example:

The BOB OFF instruction also turns off any animation or collision routines associated with these Bobs.

X BOB

function: get x-coordinate of a Bob
x-coordinate=X Bob(number)

Y BOB

function: get y-coordinate of a Bob
y-coordinate=Y Bob(number)

It is not difficult to keep track of Bobs under normal circumstances, but if Bobs are moved with AMAL, their coordinates can vary unpredictably. In which case, the X BOB and Y BOB functions may be used to get a snapshot of their current position, by returning the screen coordinates of your selected Bob. Specify the number of the chosen Bob on screen, and the appropriate coordinate will be returned, as measured from the top left-hand corner of the screen to the hot spot of the current image. For example:

AMOS Professional provides many alternative methods of moving Bobs, and each Bob can display a sequence of different images to create animation. When animating Bobs with AMAL, it is possible to loose track of the precise image currently displayed, so the next function has been supplied to rectify this.

I BOB

function: get image number used by a Bob
image=IBob(number)

I BOB returns the number of the image currently assigned to the specified Bob number. If the Bob number you want to examine does not exist, an illegal function error will be given, so it is vital to define the Bob correctly before calling I BOB. Here is an example:

GET BOB PALETTE

instruction: load image colours into current screen
Get Bob Palette
Get Bob Palette mask

This command loads the whole colour palette used for your Bobs into the current screen. A mask can be added if you like, which will load a selection of these colours only. Each individual colour is represented by one "bit" of the mask being set to a zero (off) or a one (on). Colours run from right to left, so that colour zero is represented by the bit at the right-hand end of the mask, colour 1 is second from the right, and so on. Supposing there are 16 colours in your Bob palette, you would copy the first four colours like this:

Unmasking Bobs

NO MASK

instruction: remove colour zero mask from Bob
No Mask number

A "mask" means that the background colour (colour zero) around a Bob is made transparent, so t hat the screen graphics show through. The mask is also used by certain collision detection routines. A mask is automatically set up for every Bob, and the NO MASK command takes away this mask, so that the entire Bob image is drawn on the screen, including its original background colour and any other graphics in colour zero. To remove a mask, simply use this command followed by the number of the Bob image you are interested in.

Never remove a mask from a Bob while it is being displayed on screen, or its image will be scrambled! Remember to always use the BOB OFF command first.

Bob Priority

It is important to understand that every Bob automatically possesses a priority of importance, and that this priority is based on the Bob's number. So a Bob carries a priority value from 0 to 63, and AMOS Professional uses this value to decide in which order Bobs are displayed and which Bobs barge their way in front of others when moving around the screen.

The general rule is that a Bob with a higher priority number is displayed in front of one with a lower priority number. For example, Bob 5 would cut in front of Bob 4, but be obscured if Bob 6 crossed its path. So it is clear that this priority system should always be remembered when you number your Bobs.

AMOS Professional allows changes in the priority system to suit your needs, this first system offers an alternative based not on Bob numbers, but on the position of Bobs on the screen.

PRIORITY ON

instruction: set Bob priority to highest y-coordinate
Priority On

PRIORITY OFF

instruction: set Bob priority to default status
Priority Off

When PRIORITY ON is used, Bobs with the highest y-coordinates take priority on the screen. It is usually best to set hot spots at the bottom of Bobs to exploit this priority, and some superb perspective effects can be created. All that is needed to re-set the original Bob number priorities is to use the PRIORITY OFF command.

PRIORITY REVERSE ON

instruction: toggle on Reverse Priority of Bobs
Priority Reverse On

PRIORITY REVERSE OFF

instruction: toggle off Reverse Priority of Bobs
Priority Reverse Off

The PRIORITY REVERSE ON command changes around the entire priority table based on Bob numbers. Not only does it give a lower Bob number priority over a higher Bob number, when used with PRIORITY ON it also gives priority to a Bob with the lowest y-coordinate. As you would expect, PRIORITY REVERSE OFF sets the priority system back to normal.

Bobs and screens

AMOS Professional offers a full range of commands to allow Bobs and screens to interact.

LIMIT BOB

instruction: limit bobs' visibility to part of screen
Limit Bob x1 ,y1 To x2,y2
Limit Bob number,x1,y1 To x2,y2
Limit Bob

Restricts the visibility of a numbered Blitter OBject to the LIMITs of a rectangle on screen. You set up the size of this rectangle by simply entering its coordinates.

If LIMIT BOB is followed with a Bob number, then only that Bob becomes restricted by the boundaries of the rectangle.

Note that the width of the rectangle must always be wider than the width of the Bob, and that the x -coordinates are always rounded up to the nearest 16-pixel boundary. To keep Bob number 1 trapped inside an area, you would use something like this:

Remember that a Bob must be called up with the BOB command before LIMIT BOB is used, otherwise the limitation will have no effect. To remove a Bob's visibility limit, use the command without any coordinates, like this:

DOUBLE BUFFER

instruction: activate Double Buffering system
Double Buffer

Throughout this Chapter, extensive reference is made to the technique known as "double buffering". The DOUBLE BUFFER command creates an invisible copy of the current screen and stores it as a "logical screen". All graphics operations, including Bob movements, are now performed directly on this logical screen, without disturbing your existing display at all. This is because the existing display on your television screen is taken straight from the original screen area, now called the "physical screen".

Once the image has been re-drawn, the logical screen and physical screen are swapped over. The old logical screen is flicked onto the display, and the old physical screen is hidden away to become the new logical screen. The entire process now cycles continuously, producing a solid, smooth display, even when dozens of Bobs are moving on the same screen.

Any complexities of this technique are completely automatic, so once DOUBLE BUFFER has been engaged, you can relax.

Since hardware Sprites are overlaid directly onto your television display, double buffering will hive no effect at all on any existing Sprite animations.

The double buffering system works equally well in all of the Amiga's graphics modes, and can also be used in conjunction with dual playfields. You should be aware that double buffering requires two separate areas of memory, one for the logical and one for the physical screen. So it will double the amount of memory required, for example an extra 32k will be needed for a standard 16-colour screen. This means that if you try and DOUBLE BUFFER too many screens, available memory will be exhausted.

In practice, double buffering is invaluable, and the additional memory required is well spent. It can be exploited for advanced three-dimensional routines, and is especially useful for scrolling screen effects, because the new areas of display are copied straight into the invisible background without corrupting the current display.

As an optional extra, AMOS Professional provides total control over the entire DOUBLE BUFFER system, and a full explanation may be found in the next Chapter. For a rapid insight into the effect of not using DOUBLE BUFFER, make sure you run the HELP_26 demonstration program.

That demonstration produces a horrible flickering effect. Whenever a Bob moves around the screen, the graphics beneath it are replaced at their original position. Unfortunately, since Bobs are updated at the same time as the screen images, this sort of flickering effect is generated. By including a DOUBLE BUFFER command, screens are switched after the drawing process is complete, and as explained above, the process is completely automatic.

GET BOB

instruction: grab an image from part of screen
Get Bob image,x1,x2 To x2,y2
Get Bob screen number,image,x1,y1 To x2,y2

This command grabs a selected part from the current screen and copies it straight into the Object Bank. After giving the image number to be created, set the area to be grabbed from the top left-hand corner to the bottom right-hand coordinates. If your chosen image number already exists, the existing image will be replaced by the new picture, otherwise the new picture will be added to the bank.

An optional screen number may be given immediately after the GET BOB command, allowing an image to be grabbed from a specific screen. Here is an example:

GET BOB is an extremely useful command, allowing any section of a screen to be loaded into a Bob, and then manipulated with the AMAL system. You can even write your own object editor from start to finish! It is also possible to create and modify Bob images from AMOS Professional Basic. This allows, you to produce stand-alone program listings that will run without the need for external image files. Try the next example:

PUT BOB

instruction: put a fixed copy of a Bob on screen
Put Bob number

The PUT BOB command takes the Bob whose number is given and fixes a permanent copy of its image on the screen, at the current position. This is achieved by preventing the background area beneath the Bob from being re-drawn. Note that after the image has been copied, the original Bob can be animated and moved with no ill effects.

In actual fact, PUT BOB is included as a support for STOS programmers, who wish to make their old Atari STOS programs compatible with AMOS Professional. Because it only works with single buffered screens, it is not particularly useful, and PASTE BOB is recommended as the preferred command. Please see below.

PASTE BOB

instruction: draw an image from Object Bank
Paste Bob x,y,image

PASTE BOB takes an image held in the Object Bank, and draws it straight onto the current screen. Unlike the PUT BOB command, the image is drawn immediately, so there is no need to add the WAIT VBL commands before proceeding.

It is important to note that the coordinates for the given image number are measured from the top left-hand corner of the image, and take no account of the current hot spot setting!

PASTE BOB is just like any other graphics instruction, so it does not need a double buffered screen. It can be used to generate a range of extremely fast graphical operations, and it is also useful for mapping complex displays in scrolling arcade games. Here is an example:

Bob Bank Commands

DEL BOB

instruction: delete an image from the Object Bank
Del Bob number
Del Bob first To last

The DEL BOB command permanently deletes one or more Bob images from the Object Bank. To erase a single image, simply give the image number to be deleted, like this:

Whenever an image is deleted, all the subsequent images in the Bank are moved up one place in the numerical order. For instance, if the Bank originally contained four images, the above example would remove image number 2 from memory, leaving a gap between images 1 and 3. This gap would be filled immediately, as the old image numbers 3 and 4 were shunted up one place, to become the new image numbers 2 and 3.

If more than one image is to be removed from the Bank, you can set the range from the first image to the last after a DEL BOB command. The following example would delete Bob images 4,5,6 and 7:

After the last image has been deleted from the Object Bank, the entire Bank is erased automatically.

INS BOB

instruction: insert a blank Bob image into the Object bank
Ins Bob number
Ins Bob first To last

INS BOB inserts a blank image at the numbered position in the current Object Bank. All of the images after this numbered position will then be moved down one place in the numerical order. The second version of this command allows you to create several spaces in a single operation, by giving the range of new gaps between the first and last image numbers that you specify.

Any of these new image spaces are completely empty, and so cannot be allocated to a Bob or displayed directly on screen while they are still blank. An actual image must first be grabbed into the Object Bank, using a GET SPRITE or GET BOB command. If this is not done, the appropriate error message will be given as soon as you try to access the empty image.

Both DEL BOB and INS BOB are provided to be used with the GET BOB and GET SPRITE commands. They allow you to modify and adjust your Bob images from inside AMOS Professional programs, with complete freedom. They may be used to create numerous special effects such as interactive screen animations and animated brushes, as used in Deluxe Paint.

Flipping Bob Images

AMOS Professional is designed to meet every programming need when it comes to animating images. You will often need to animate mechanical objects and cartoon characters as realistically as possible, so every movement sequence must be created from a number of images, and each image in the sequence must be carefully drawn using the Object Editor, ready for smooth animation with AMAL.

Unfortunately, perfectly animated sequences need a great many images, which take up a great deal of memory. To move the animated character in several directions makes the problem much worse, because each direction needs a separate sequence of images.

AMOS Professional cuts such waste of memory to a minimum. This is achieved by allowing you to display the same image in different orientations, so that a character can be mirrored and turned upside down, simply by flipping its image.

HREV

function: flip an image horizontally
new number=Hrev(image number)

This function reverses an image from left to right, creating a mirror image. Use HREV by specifying the existing image number (in brackets) to be flipped horizontally, in order to create a new identification number for the reversed image. This new image number can be freely used with any of the standard Bob commands.

Here is an example:

There is a hexadecimal version of this function, and the value returned by the HREV function is in the following format:

$800+n

Where $8000 is a "flag" telling AMOS Professional to reverse the Bob whenever it is displayed on screen, and where n is the number of your image. This technique can be used to flip images directly from an AMAL animation sequence.

Supposing your original sequence was created with this:

To reverse these images, either of the following two lines could be used:

When an image is reversed like this, the location of the hot spot is reversed horizontally too. So if the hot spot was originally in the top left-hand corner, the hot spot of the HREV image will be in the top right-hand corner: Depending on the image involved, this can have a great effect on the way your image is displayed on screen. Be careful to position your hot spots sensibly, or avoid any risks by setting them centrally, using the appropriate HOT SPOT command.

VREV

function: flip an image vertically
new number=Vrev(image number)

VREV is identical to HREV, except that it takes the specified image and turns it upside down before displaying it on the screen. This is best used for animated objects that move vertically, although comic effects can be achieved with cartoon characters.

As with HREV, there is an equivalent hexadecimal version of the VREV function, which can be used with AMAL animation strings. The format is:

$4000+n

Where $4000 acts as the reversal flag, and n is the image number. Here are two typical AMAL string of reversed animation:

REV

function: double-flip an image vertically and horizontally
new number=Rev(image number)

REV combines HREV and VREV into a single function. It takes the image whose number is held in brackets, reverses it from left to right and then performs another reversal from top to bottom. For example:

Don't forget to try the HELP programs for a demonstration. If your own attempts at flipping Bob images cause any problems, you may wish to consult the Bob Doctor, below.

The Bob Doctor

Here are some free consultations which answer common problems encountered when flipping Bobs.

Problem: When I use flipped Bobs on screen with their original images, my display slows down to a crawl.
Remedy: Do not display the same image in different orientations on screen at the same time. AMOS Professional flips images during the updating process, just before Bobs are re-drawn on screen. Once reversed, images stay in this new state until displayed in a different direction. Whenever AMOS Professional flips a reversed image, it first needs to restore the image to its original state. This takes a great deal of processor time, and slows down your display.

Problem: Can I reverse an image for later use, without displaying it on screen?
Remedy: Yes. PASTE BOB works perfectly with flipped images, and can be used directly with HREV, VREV and REV. If you want to reverse an image quickly, without displaying a Bob, try something like this:

Since the coordinates lie outside of the current screen area, the image is not displayed, but it is still flipped by the PASTE BOB command.

Problem: I want to flip my Sprites as well as my Bobs?
Remedy: The flip functions do not work with Sprites directly, but there is no problem in displaying a flipped Bob image as a Sprite. This line would be completely ignored:

But the following routine will solve your problem:

Problem: Can I check for a collision between two copies of the same image, for example, between an original image and its own mirror-image?
Remedy: Yes, but it is not recommended. If the image's hot spot has been centred the results should be acceptable, but if the hot spot is asymmetrical you will generate unpredictable problems.