Speeding up animation as velocity increases???

When moving a chopper picture around the screen using 3 different chopper pictures, how does one increase the animation speed as the chopper speed increases in velocity?

The drawHelicopter function below switches between each helicopter picture using a counter and then resets. The move function is included to show you how I'm increasing the chopper's speed.

Code:

void Helicoptor::drawHelicopter(MyDrawEngine* pTheDrawEngine, MyPicture* pHelicopter, double fRate)
{
	sourceRect.top 		= 0;
	sourceRect.bottom 	= 32;
	if(frameCounter == 0)
	{	
		sourceRect.left		= 0;
		sourceRect.right	= 64;
	}
	if(frameCounter == 1)
	{
		sourceRect.left = 64;
		sourceRect.right = 128;
	}
	if(frameCounter == 2)
	{
		sourceRect.left = 128;
		sourceRect.right = 192;
	}

	destRect.top = (int)yCoord;
	destRect.bottom = (int)yCoord+32;
	destRect.left = (int)xCoord;
	destRect.right = (int)xCoord+64;

	//Blit: Copy from surface to back buffer(temp draw surrface)
	pTheDrawEngine->Blit(destRect, sourceRect, pHelicopter);
}

void Helicoptor::move(MyInputs* pTheInputs, double fTime)
{
	yCoord = yCoord + yVelocity * fTime;
	xCoord = xCoord + xVelocity * fTime;

	if(yCoord < 0) yCoord = 0;
	if(yCoord > 720) yCoord = 720;
	if(xCoord < 0) xCoord = 0;
	if(xCoord > 960) xCoord = 960;
	
	//Move direction
	if(KEYPRESSED('Q') && (yVelocity > -61))
	{
		yVelocity = yVelocity - pixels_per_sec;
	}
	if(KEYPRESSED('A') && (yVelocity < 61))
	{
		yVelocity = yVelocity + pixels_per_sec;
	}
	if(KEYPRESSED('N') && (xVelocity > -61))
	{
		xVelocity = xVelocity - pixels_per_sec;
	}
	if(KEYPRESSED('M') && (xVelocity < 61))
	{
		xVelocity = xVelocity + pixels_per_sec;
	}

	frameCounter++;
	if(frameCounter == 3) frameCounter = 0;
}

One option is to store the frameCounter as a decimal number instead. Each frame you increase it in the lines of:

Code:

frameCounter += (xVelocity * xVelocity + yVelocity * yVelocity) * fTime * someConstant;

The (xVelocity * xVelocity + yVelocity * yVelocity) is to calculate the squared length of the speed vector, or how fast the helicopter is going (squared speed). When you test what frame to draw you simply cast it to an int.

Another solution is to update the animation each frame, with the delta time from the last frame. This value gets added to a variable that keeps track of how long it has been since the last frame, if this variable is greater than the 1/FPS of the animation you increase the frame.
Something in the lines of: (pretty much uncommented code, but it should be pretty clear)

Code:

AnimationController::VUpdate(float delta)
{

	if (m_pCurrentAnimation)
	{
		m_TimeSinceLastFrame += delta;

		while (m_TimeSinceLastFrame > 1.0f/m_pCurrentAnimation->fps)
		{
			m_CurrentFrame++;

			//Check if the current frame is at the end of the animation.
			if (m_CurrentFrame >= m_pCurrentAnimation->start + m_pCurrentAnimation->length)
			{
				m_CurrentFrame = m_pCurrentAnimation->start;

			}

			m_TimeSinceLastFrame -= 1.0f/m_pCurrentAnimation->fps;
		}
	}

}

Good luck!

The problem here is that you are always incrementing the animation once very update. So in this sense it is always animating as fast as possible. You'll need to put in code like Wazaa's to make it update based on time, not on frames. So if the velocity is 0 you shouldn't increment the animation at all, if it is say 2 you might increment the animation every 8 updates. If the helicopter is booking then you would increment every update like you are now.

edit:
I'm bored so I figured I would write this.
ex.
replace

Code:

	frameCounter++;
	if(frameCounter == 3) frameCounter = 0;

with

Code:

        //m_animationTimer is a member variable of Helicopter
	m_animationTimer -= fTime * sqrt(pow(yVelocity,2)+pow(xVelocity,2));//gets magnitude
	if( m_animationTimer <= 0 )
	{
		frameCounter++;
		if(frameCounter == 3) frameCounter = 0;
		//m_ANIMATION_TIMER_START is a constant that you should tweak.
		m_animationTimer = m_ANIMATION_TIMER_START;
	}

Ha ha thats quality that guys. Thanks.

P.s. now I feel guilty for not doing it myself.