Does 1 big image = slower performance, even if only 10% of it is drawn?

Question

Hey, I want to make a tile based game, but I was wondering... If I created a large image thats bigger than the game window based on the tile data, would it be slow to draw this image in the window, even if only %25 of the image is ever seen due to the smaller size of the window? (Is it automaticaly clipped?)
Or is it better to just draw the current viewable tiles over and over instead of creating one large image?
In case your wondering, this is for a RTS game, that will involve the world map being scrollable.

Answer 1

In a nutshell, yes.
As you have written it, Java will load the entire image into memory and treat the resulting block of memory as one "continous block" as far as operations go. If you decided to clip 25% of the image, you will have to write a routine that determines the boundaries of the visible region, and Java then will have to determine the memory addresses of that region and copy the relevant details to the buffer. In other words, you still have to calculate what region to display.
(I'm glossing over the details, I just want to give you enough background so that you can make an informed decision based on your needs.)
Java does automatically clip if you tell it to draw two overlapping regions but you still have to specify which region has priority.
The alternative is to break the image down into tiles. Each tile likewise takes up a chunk of memory but you can eliminate all redundant tiles. That is, if you want to show a 5x5 square of grass, you can instruct the game to draw the 1x1 square of grass 25 times as opposed to the one 5x5 tile, and the smaller tile takes up less memory. More importantly, you can choose to update only the relevant tiles as the player travels across the map. I.e., player moves left. Draw new tile in spot player just vacated. Leave rest of map untouched.
So to answer your question, which approach is better depends on exactly what you want to do and what your hardware constraints are. Some people would even suggest mixing and matching, using the clipping feature to draw the player tile on top of the visible portion of the map.
A hint: decide whether you want the player to remain stationary while the world scrolls underneath him (use tiles) or if you want the player to move around while the world stays stationary (use large image).