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Screen fade, blanking instead of fading.

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#1 mathew_653   Members   


Posted 24 January 2013 - 05:47 PM

Hello, i have looked at the other subforms of this forum and feel this is the best place.

I shall first off explain my issue and my own observations, before showing the relivent code.


The goal is to create a fade out effect.

At first i suspected multithreading, i have seen weird stuff happen in the past with it.


But this has prooved not to be the case, the float being past appears to be incrementing at the correct rate, but where the fault occurs is that instead of a fade, i get a flick of the screen then blankness as if the fade is at 100% when it is not(the printf in fade drawing showed that).


Additional infomation, the platform i am using is windows but i use SDL to handle all the initalising and stuff, as i do plan to port the code later to other platforms.


I shall now quote my code, starting with relivent render code(Cut down for cleaness):



				_GL_Color ClearColor = RGBtoFloat({0,0,0,255});
				glClear( GL_COLOR_BUFFER_BIT );
				glEnable( GL_CULL_FACE );

				//Enable alpha masking here for non background element drawing.

				glEnable( GL_BLEND );
				glEnable( GL_ALPHA_TEST );

				//Set to no texture
				glBindTexture(GL_TEXTURE_2D, 0);
				if (pFade != NULL) { pFade->Draw(); }
				glDisable( GL_ALPHA_TEST );
				glDisable( GL_BLEND );
				glDisable( GL_CULL_FACE );


Fade draw function(inside fade class)

	if (Colors[4] <= 0.0f) { return; }

	printf("Colors[4] = %f\n", Colors[4]);
	glBegin( GL_QUADS );
	//Bottom left
	glColor4f( Colors[1], Colors[2], Colors[3], Colors[4] );
	glVertex3f( 0, ScreenY, 0.0f );

	//Bottom right
	glColor4f( Colors[1], Colors[2], Colors[3], Colors[4] );
	glVertex3f( ScreenX, ScreenY, 0.0f );
	//Top right
	glColor4f( Colors[1], Colors[2], Colors[3], Colors[4] );
	glVertex3f( ScreenX, 0, 0.0f );

	//Top left
	glColor4f( Colors[1], Colors[2], Colors[3], Colors[4] );
	glVertex3f( 0, 0, 0.0f );


In advance, thank you for any help you can provide.

#2 LorenzoGatti   Members   


Posted 25 January 2013 - 02:29 AM

As always, complete code samples would be better:
  • What do you put into Colors[]? How is it animated?
  • What's the point of GL_ALPHA_TEST? Don't you want to draw arbitrary stuff first, and an increasingly opaque rectangle over whatever you are fading out last, unconditionally? Every fragment should be a*fade colour + (1-a)*stuff colour, with a (Colors[4] for you) increasing from 0 to 1 and ignoring the alpha value of the fading stuff.
  • When do you draw the stuff you are fading out, and in which OpenGL state? I only see pFade->Draw(), which I suppose contains only the rectangle in the second snippet.

Edited by LorenzoGatti, 25 January 2013 - 10:35 AM.

Omae Wa Mou Shindeiru

#3 SimonForsman   Members   


Posted 25 January 2013 - 04:37 AM

It looks like you're only clearing the color buffer, never the depthbuffer and from what i can see depth writes or depth tests are never disabled, so once the fade quad is rendered at z=0.0 everything else should fail the depth test until you clear the depthbuffer.


If that isn't the problem it will help a lot if you post a minimal, compiling example demonstrating the problem.

I don't suffer from insanity, I'm enjoying every minute of it.
The voices in my head may not be real, but they have some good ideas!

#4 mathew_653   Members   


Posted 25 January 2013 - 08:40 AM

Gonna get on with decuppleing the code to produce a minimal build example.
But first i will clarify some more on my code.

Colors[] defined as
float Colors[4];
it is initalised in CEffectFade's constructor.

pAppState is a pointer from the main thread, passed as an argument.

It is incremented from a saperate thread, like so(excuse the mess, the project is still prealpha and i have been tinkering around with stuff).
	const int TICKS_PER_SECOND = 950;
	const int SKIP_TICKS = 1000 / TICKS_PER_SECOND;
	const int MAX_FRAMESKIP = 5;
	DWORD NextFrame = timeGetTime();	//Get our current timeing.	
	DWORD LastFrame = timeGetTime();	//
	int loops=0;
	float interpolation=0.0f;
	while (pAppState->status & STATUS_ACTIVE)	{
		CPlayerEntity *pPlayer = (CPlayerEntity *)pAppState->SharedTable[ID_PLAYER];
		CEffectFade *pFade = (CEffectFade *)pAppState->SharedTable[ID_OVERLAY];
		CLoadAnimation *pLoadAnim = (CLoadAnimation *)pAppState->SharedTable[ID_LOADANIMATION];
		if (timeGetTime() > NextFrame && loops < MAX_FRAMESKIP)	{
			if (pAppState->status & STATUS_LOADING)	{
				if (pLoadAnim != NULL)	{
				if (pPlayer != NULL) { pPlayer->Think(); }
					NextFrame += SKIP_TICKS;
			if (abs(LastFrame-NextFrame) > 1) {
				printf("Warning : delta is %i, game might be lagging.\n", abs(LastFrame-NextFrame));
			LastFrame = NextFrame;
			//Give our time to something else.
		interpolation = (float) timeGetTime() + SKIP_TICKS - NextFrame / (float)SKIP_TICKS;
		pAppState->Interp = interpolation;
		//printf("Interpolation : %i\n", interpolation);
		// Old approach(Replaced with a high resolution timer)
Two two refrenced functions here SetRate and TransitionIn are as so(raterate is a float).
void CEffectFade::TransitionIn(){
	//printf("Fade : %f\n", Colors[4]);
	Colors[4]+= RateRate;
	if (Colors[4] > 1.0) { Colors[4] = 1.0; }
	if (Colors[4] < 0.0) { Colors[4] = 0.0; }
void CEffectFade::SetRate(float Rate){	RateRate = Rate;}
I removed GL_ALPHA_TEST while cleaning code up.

I draw the screen and map prior to the call to pfade->draw(); inbetween the glEnable block and the glBindTexture(GL_TEXTURE_2D, 0) call.

Edited by mathew_653, 25 January 2013 - 08:49 AM.

#5 __SKYe   Members   


Posted 25 January 2013 - 08:25 PM

Okay, first, like SimonForsman said, add the GL_DEPTH_BUFFER_BIT flag to the glClear() function.


Then, instead of trying the fade out effect, try to render the quad at half alpha (like the line you commented on the last snippet), and see if it renders correctly.

If it does, then the problem is in your rate update. If it does not, then maybe transparency is not being applied correctly, so when the quad as an alpha > 0.0, then if renders full opaque (not sure, just an idea).


Also, i'm not sure, but don't you have to enable alpha blending & depth testing for transparency to work?


See if it helps.

#6 mathew_653   Members   


Posted 27 January 2013 - 01:47 PM

An update, i have found a solution to the issue, i was having altough, it is a issue i had not encountered for ages, relating to transparancy and glColor4f using 0,0,0 for color.

I have tested my code with clolor 1.0,1.0,1.0 and found it worked fine, so that isolated my fault downto an old thing that i have seen with glColor4f(0,0,0,A); and alpha blending.



Altough i did try all the ones, prior, i thank you kind people for your assistance, i hope mentioning this, prevents anyone else being cought out.

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