6.1 Format of the VIEW resource by Peter Kelly <ptrkelly@ozemail.com.au> Last updated: 5 October 1997 Overview View resources contain some of the graphics for the game. Unlike the picture resources which are full-screen background images, view resources are smaller 'sprites' used in the game, such as animations and objects. They are also stored as bitmaps, whereas pictures are stored in vector format. Each view resource consists of one or more 'loops'. Each loop in the resource consists of one or more 'cels' (frames). Thus several animations can be stored in one view, or a view can just be used for a single image. The maximum number of loops supported by the interpreter is 255 (0-254) and the maximum number of cels in each is 255 (0-254). View header (7+ bytes) Note: ls,ms means that the value is a two-byte word, with the least significant byte stored first, and the most significant byte stored second, e.g. 12 07 is acually 712 (hex) or 1810 (decimal). Most word values in AGI are stored like this, but not all. Byte Meaning 0 Unknown (always seems to be either 1 or 2) 1 Unknown (always seems to be 1) 2 Number of loops 3-4 Position of description (more on this later) (ls,ms) Both bytes are 0 if there is no description. 5-6 Position of first loop (ls,ms) 7-8 Position of second loop (if any) (ls,ms) 9-10 Position of third loop (if any) (ls,ms) ....   Note: Two of these loop references CAN point to the same place. This is done when you want to use mirroring (more on this later). Loop header (3+ bytes) Byte Meaning 0 Number of cels in this loop 1-2 Position of first cel, relative to start of loop (ls,ms) 3-4 Position of second cel (if any), relative to start of loop (ls,ms) 5-6 Position of third cel (if any), relative to start of loop (ls,ms) Cel header (3 bytes) Byte Meaning 0 Width of cel (remember that AGI pixels are 2 normal EGA pixels wide so a cel of width 12 is actually 24 pixels wide on screen) 1 Height of cel 2 Transparency and cel mirroring The first four bits of this byte tell the interpreter how to handle the mirroring of this cel (explained later). The last four bits represent the transparent color. When the cel is drawn on the screen, any pixels of this color will show through to the background. All cels have a transparent color, so if you want an opaque cel then you must set the transparent color to one that is not used in the cel. Cel data The actual image data for each cel is stored using RLE (run length encoding) compression. This means that instead of having one byte for each single pixel (or 1/2 byte as you would use for 16 colors), each byte specifies how many pixels there are to be in a row and what colour they are. I will refer to these groups of pixels as 'chunks'. This method of compression is not very efficient if there is a lot of single pixels in the image (e.g. a view showing static on a TV screen), but in most cases it does save a fair amount of space. Each line (not to be confused with a chunk) in the cel consists of several bytes of pixel data, then a 0 to end the line. Each byte of pixel data represents one chunk. The first four bits determine the colour, and the last four bits determine the number of pixels in the chunk. e.g. AX BY CZ 00 This line will have: X pixels of colour A [AX]   Y pixels of colour B [BY]   Z pixels of colour C [CZ] (then that will be the end of the line) [00] If the color of the last chunk on the line is the transparent color, there is no need to store this. For example, if C was the transparent color in the above example, you could just write AX BY 00. This also saves some space. Mirroring Mirroring is when you have one loop where all the cels are a mirror image of the corresponding cels in another loop. Although you could do this manually by drawing one loop and then copying and pasting all the cels to another loop and flipping them horizontally, AGI views provide the ability to have this done automatically - you can draw one loop, and have another loop which is set as a mirror of this loop. Thus, when you change one loop you change the other. This is useful if you have an animation of a character walking left and right - you just draw the right-walking animation and have another loop a mirror of this which will have the left-walking animation. Another advantage of cel mirroring is to save space - it doesn't make much difference these days, but back when AGI was written the games were designed to run on 256k systems which meant that memory had to be used as efficiently as possible. Mirroring is done by having both loops share the same cel data - you saw above that you can have two loop references pointing to the same place. The first four bits of the 3rd byte in the header of each cel tell the interpreter what is mirrored: Bit 1 specifies whether or not this cel is mirrored. Bits 1,2 and 3 specify the number of the loop (from 0-7) which is NOT mirrored. When the interpreter goes to display a loop, it looks at the bit 1 and sees if it is mirrored or not. If it is, then it checks the loop number - if this is NOT the same as the current loop, then it flips the cel before displaying it. Leaving enough room for the mirrored image: If you have a cel that is mirrored, you need to ensure that the number of bytes the cel takes up in the resource is greater than or equal to the number of bytes that the flipped version of the cel would take up. The reason for this is that the interpreter loads the view resource into memory and works with that for displaying cels, rather than decoding it and storing it in memory as a non-compressed bitmap. I assume that it doesn't even bother 'decoding' it as such - it probably just draws the chunks of color on the screen as they are. When it has to display the flipped version of a cel, instead of storing the flipped cel somewhere else in memory, it flips the version that is there. So in memory you have the view resource that was read from the file, except that some of the cels have been changed. This is why there is mirroring information stored in each cel - the interpreter has to know what cels have been changed. When it flips a cel, it changes the loop number in the 3rd byte of the cel header to the number of the loop it is currently displaying the cel for. So when it looks at this number the next time for a different loop, it will see that the cel is not the right way round for that loop and mirror it again. This process seems very inefficient to me. I don't know why it doesn't just draw the chunks of color on the screen back to front. But since it does it this way we have to make sure that there is enough room for the flipped cel. It seems that not all versions of the interpreter require this, however - I was working with version 2.917 when I was testing this out, but I noticed that versoin 2.440 did not require this. I will attempt to try this with all different interpreters and provide a list of which ones use this in the next version of this document. But it is best to put these bytes in just in case, as the views will still work regardless. Description All the Views in the game that are used as close-ups of inventory items have a description. When a player 'examines' these (in some games you can select 'see object' from the menu), they will see the first cel of the first loop of this view and the description of the object they are examining. This is brought up using the show.obj command. The Description is stored in plain text, and terminated by a null character. Lines are separated by an 0x0A.