CompuServe Thread

I hope this will be of some use: >> 1) After opening a Custom Screen, is it advisable to use a Backdrop Window to render the game graphics into, or better to render the game graphics right to the screens memory? There are several methods you could use on the Amiga, depending on how much graphics you need to shift around. If you need to copy large images to the screen, then a buffer of some kind will be essential, because you won't have had time to draw all of the bitplanes before the screen is refreshed – and it will be noticeable! When you have your complete screen in the buffer, either a) use the blitter to copy the changes into the screen ram, or b) change the bitplane pointers to point to your buffer, and build the next frame in the memory that was the old screen. Method a) will only work if there is not too much to update. >> 2) What about the use of the blitter and copper in the above situations? The blitter is extremely useful for copying blocks of graphics around – but it isn't quick enough to copy the entire screen 50/60 times a second. (If anyone thinks it is – let me know!). It is best used to move individual images between buffers & main memory. You can in theory use the processor while the blitter is working – it can generate an interrupt which you can use to set it up for the next bitplane. The blitter can also shift images left or right by up to 16 pixels – an operation which costs nothing in terms of extra time and which allows you to place your graphics at pixel positions with ease. You can get the copper to cause an interrupt on specific scan lines – perhaps to change the color palette or as I do to cause an interrupt on the last scan line to give a longer "vblank" period. >> 3) I've noticed in the source code to several Amiga and PC games, that a file is included that is usually a full screen of various graphics..etc.. You can't use these graphics directly as sprites, if the file is actually arranged as a normal screen. But you can copy the data from the file into an area of chip ram reserved by you for your sprites. I usually use a program to chop individual images out of picture files and store them in a format that the blitter can use directly (ie in bitplanes with a mask, and no compression). I think time spent preparing your graphics and getting them into a form more easily used by the program is well spent. >> 3) (4?)Scrolling backgrounds/foregrounds. How are they done and what exactly is tileing and how is it done. Scrolling is much easier on an Amiga than many other machines. You need to set up a screen that is wider than the part you can actually see, and then you can use the hardware scroll registers to shift the screen horizontally by up to 16 pixels – thus bringing the hidden part into view. To scroll up or down, you just change the bitplane pointers to point to the row above or below. You can use the copper to generate interrupts to allow the bitplane pointers to be changed when you reach the end of screen memory to wrap round to the top – thus allowing you to scroll continuously in one direction, without running out of ram. Tileing is when you design your graphics so that they can be cut up into rectangular 'tiles' which can be placed next to each other to form a complete screen – often the same tile is used many times in one screen. You specify the appearance of the screen by having a 'map' which is basically a table showing which tiles go where. Using this method cuts down on the amount of drawing that needs to be done to construct a level, and saves a lot of memory, but too few tiles will make your screens look repetitive and blocky. 4 – 10 – sorry, these are not things I know much about. Try the various game forums and the IBM forums for info on how Doom was done – there are many helpful files available. 11) The use of interupts in game programming, am I correct in assuming that these are used so that sound and graphics may be handled togther? If not, can you give me some insight into their use in this type of application? Interrupts allow things that need attention (like the sound, disk blitter etc) to stop the main program and get the service they need. For example, when the display has been completely refreshed, the electron beam is at the bottom of the screen. The time when the beam is moving back to the start position would be useful for drawing graphics without affecting the picture. The video system creates an interrupt which makes the computer stop what its doing and switch to another function called an interrupt handler or interupt service routine. This routine could draw graphics, increment a timer, check the mouse, keyboard or all of these things. The sound system creates interrupts to allow you to set up the sound pointers for the next chunk of sound. Obviously it would be difficult to regularly check whether a sound had finished – you may miss it. The interrupt system takes these worries away and allows your program to concentrate on its main task. >> 12) Books dealing with game programming, the techniques employed, algorithms and such, and books on programming computer graphics in general,could I get some good suggestions here. For Amiga programming, you will need two essential books: 1. Amiga Hardware reference manual – published by Addison Wesley. This tells you how to use all the hardware – sprites, copper, blitter etc. It is very detailed, but not difficult to understand. 2. Amiga ROM Kernel reference manual: Includes and Autodocs – also published by Addison Wesley. This contains details of all the functions available in the operating system for doing things like disk IO, memory allocation, device handling etc. Its a must if you wish to use assembler for these kinds of tasks. Its a bit tricky to get into, but using the operating system functions not only allows you to write code that is compatible across the whole amiga range, but it will save you time because you won't need to write so many of your own routines. Many games programmers avoid the operating system altogether because they think it will slow their games down. This is not true – its an excuse not to bother learning a better way to go about things. Finally – I would suggest you keep your posts to no more than a couple of questions at a time. Seeing a huge pile of queries puts people off from giving any answer at all. I only took the (considerable) time to reply because I was stuck with a bug and wanted a break from it. If you keep your questions short, you are more likely to get a selection of replies from people with different experiences – much more useful! Mal "With writers cramp in Montreal"