As you can see from the illustration, then the upper-left X position of a frame is equal to the FrameNumber ?1 multiplied by the width of the sprite. So by using this scheme we move the X position by an amount which is equal to the width of the sprite, on each update of the frame.
FrameNumber = (FrameNumber Mod MaxFrames) 1
This actually ensures that the FrameNumber variable will never be more than MaxFrames and never less than 1. The method also produces a problem, since when we use it to get the X positions we will get values in the range from 64 (Frame 1) to 640 (Frame 10). These values represent the rightmost X position of the sprites, which is of no use to the BitBlt function. The solution is of course to simply subtract 1 from the FrameNumber when we calculate the X position, and thereby getting X values in the range of 0 to 576, or all the leftmost positions of the sprite frames.
The same thing could be employed to keep the sprite from moving out of bounds of the window:
X = (X Mod Me.ScaleWidth) 1
Y = (Y Mod Me.ScaleHeight) 1
So now that we have all this wonderful background information, it should be simple to implement in a timer event and draw the thing:
Private Sub TimerAnimation_Timer()
Static X As Long
Static Y As Long
''''Clear the form, since we do not have a background
Me.Cls
''''Draw the mask
BitBlt Me.hDC, X, Y, SpriteWidth, SpriteHeight, picMask.hDC, _
(FrameNumber - 1) * SpriteWidth, 0, vbSrcAnd
''''Draw the sprite
BitBlt Me.hDC, X, Y, SpriteWidth, SpriteHeight, picSprite.hDC, _
(FrameNumber - 1) * SpriteWidth, 0, vbSrcPaint
''''Update frame number
FrameNumber = (FrameNumber Mod MaxFrames) 1
''''Update drawing positions
X = (X Mod Me.ScaleWidth) 1
Y = (Y Mod Me.ScaleHeight) 1
''''Force an update of the form
Me.Refresh
End Sub
We employ the usual scheme of first drawing the mask and then the actual sprite. As you can see from the picture box with the masks, we have created a mask for all the sprites, and draw these mask frames in the same manner as the sprites. This would not be necessary in real life with this particular sprite example. Since each frame of this sprite''''s animation sequence is the same shape and therefore has the same transparent and visible areas we could have used the same mask for each circle.
Run the project and press the Start button. Observe how the sprite changes color as it moves down over the form.
More on Timing
This is all fine and good, but you may run into a situation where you do not want the same frame change (rate) as the timer interval. For example let''''s imagine that you have created a scene with some slow blinking lights which you want to blink once every second. If you were to blink the lights using the game loop, which we''''ll say is firing once every 20 milliseconds, the lights would blink so fast you wouldn''''t be able to see it. The problem here is to delay the blinking of the lamp, so it will only blink, or Blit, once each second and not every time the game loop is fired. That is why we will use the GetTickCount API, to check if a second has elapsed, and if it has, fire the changing of the lamp. The GetTickCount() function returns the number of milliseconds since Windows was started. By calling this function and using it to check the elapsed time since a frame was last updated, we can determine whether or not it is time to change the frame. For this task we need two variables, one to keep track of the current elapsed time, and one to keep track of the time since the last frame was updated. In the the ANIMATION project these are declared as LastTick (for the last time since a frame was updated) and CurrentTick (for the current time). A constant, representing the defined time we want between each frame update, is also needed. In the project the constant FrameTime is used for this. It is set to a value of 1 second.
The process of determining whether frame should be updated or not, is very simple. We first get the current time and store it in the CurrentTick variable. Then the time since the last frame update is subtracted. The result is then compared against the defined interval between the frames and if it is greater, the frame is updated. In the code if would look like this (The bold areas):
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