efficent scroll with SDL and openGL

tanoatlq · 08-27-2008, 02:43 AM

Hello,
I have an SDL surface representing the audio of a song.
I would like to show on the screen the current portion that is played
as a cursor on the middle and scroll it while music goes on.
I think some openGL function could help me.
Suggestions?
Thanks,
tano

resetreset · 08-27-2008, 05:05 AM

i have no idea about the answer to your qusetion, but just wanna say that there are some opengl tuts on nehe.gamedev.net .

dmail · 08-27-2008, 05:42 AM

The effect you are talking about is referred to as scrolling see this page for information about it. Using OpenGL you want to adjust the GL_MODELVIEW matrix so that the correct portion of the background is in the buffer.

edit:Sorry I forgot the title of the thread

ErV · 08-27-2008, 07:13 AM

Quote:

Originally Posted by tanoatlq

Hello,
I have an SDL surface representing the audio of a song.
I would like to show on the screen the current portion that is played
as a cursor on the middle and scroll it while music goes on.
I think some openGL function could help me.
Suggestions?
Thanks,
tano

It's very easy to do.

Either build surface for entire song and scroll it, or draw surface only for currently visible segment.

Code:

#include <SDL/SDL.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <wchar.h>
#include <math.h>

const double pi = 3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798214808651328230664709384460955058223172535940812848111745028410270193852110555964462294895493038196442881097566593344612847564823378678316527120190914564856692346034861045432664821339360726024914127372458700660631558817488152092096282925409171536436789259036001133053054882046652138414695194151160943305727036575959195309218611738193261179310511854874462379962749567351885752724891227938183011949129833673362; //:-) 
static int scrWidth = 640;
static int scrHeight = 480;

float song(float pos){
	float result = 0.0f;
	const int numHarmonics = 5;
	for (int i = 0; i < numHarmonics; i++){
		float f = (1 << i);
		result += sinf(pi*pos*f);
	}
	result /= (float)numHarmonics;
	return result;
	//return sinf(pi*pos) + sinf(pi*;
}

int main(int argc, char** argv){
    SDL_Init(SDL_INIT_VIDEO);
	
	SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
	SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
	SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
	SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 16);
	SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
	
	if (SDL_SetVideoMode(scrWidth, scrHeight, 32, SDL_OPENGL) == 0){
		fprintf(stderr, "couldn't set mode %dx%d!\n", 640, 480);
		SDL_Quit();
		return -1;
	}
	
	SDL_ShowCursor(SDL_DISABLE);
    
	glClearColor(0.2, 0.2, 0.2, 0.2);
	glClearDepth(1.0);
	
	glEnable(GL_DEPTH_TEST);
	
	bool running = true;
	Uint32 lastTick = SDL_GetTicks();
	Uint32 curTick = lastTick;
	while (running){
		SDL_Event event;
		if (SDL_PollEvent(&event)){
			switch(event.type){
				case SDL_QUIT:
				    running = false;
				    break;
			};
		}
		
		curTick = SDL_GetTicks();
		float delta = float(curTick - lastTick)/1000.0f;
		lastTick = curTick;
		
		glMatrixMode(GL_PROJECTION);
		glLoadIdentity();
		gluOrtho2D(0, scrWidth, scrHeight/2.0f, -scrHeight/2.0f);
	
		glMatrixMode(GL_MODELVIEW);
		glLoadIdentity();
		static float offset = 0;
		glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
		glBegin(GL_LINE_STRIP);
		glColor3f(0, 1, 0);
		offset += delta;
		if (offset > 3.0f)
			offset -= 3.0f;
		float lineOffset = offset - 0.5f;
		float cursorPos = 0.5f;
		if (lineOffset < 0){
			cursorPos = lineOffset+0.5;
			lineOffset = 0;
		}
		if (lineOffset > 2.0f){
			cursorPos = lineOffset - 1.5f;
			lineOffset = 2.0f;
		}
		for (int x = 0; x < scrWidth; x ++)
			glVertex2f(x, scrHeight*song(((float)x/(float)scrWidth)+lineOffset)*0.5f);
		    
		glEnd();
		glBegin(GL_LINE_STRIP);
		glVertex2f(cursorPos*scrWidth, scrHeight*0.5f);
		glVertex2f(cursorPos*scrWidth, -scrHeight*0.5f);
		glEnd();
			
		glDisable(GL_DEPTH_TEST);
		
		glFlush();
		SDL_GL_SwapBuffers();		
	}	
	
    SDL_Quit();
    return 0;
}

With surface it'll be same thing.

--EDIT--
By the way, you can't render SDL_surface with OpenGL. Either make OpenGL texture from that surface and render it, or don't use OpenGL and use SDL_BlitSurface function.

tanoatlq · 08-27-2008, 07:20 AM

Thanks!
I have to look at it one minute, but it seems not to be complex.

ErV · 08-27-2008, 07:53 AM

With texture:

Code:

#include <SDL/SDL.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <wchar.h>
#include <math.h>

const double pi = 3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798214808651328230664709384460955058223172535940812848111745028410270193852110555964462294895493038196442881097566593344612847564823378678316527120190914564856692346034861045432664821339360726024914127372458700660631558817488152092096282925409171536436789259036001133053054882046652138414695194151160943305727036575959195309218611738193261179310511854874462379962749567351885752724891227938183011949129833673362; //:-) 
static int scrWidth = 640;
static int scrHeight = 480;
GLuint texture = 0;

GLuint genTexture(){
	GLuint tex = 0;
	glGenTextures(1, &tex);
	glBindTexture(GL_TEXTURE_2D, tex);
	glPixelStorei(GL_PACK_ALIGNMENT, 1);
	const int width = 256;
	const int height = 16;
	Uint32 data[height][width];
	for (int x  = 0; x < width; x++)
		for (int y = 0; y < height; y++){
			int color = (x & 1) ^ (y & 1);
			Uint32 color1 = 0xFF000000|(x << 16);
			Uint32 color2 = 0xFF000000|y << 4;
			data[y][x] = color ? color1: color2;
		}
	
	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);
	
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);//_MIPMAP_LINEAR);
	glBindTexture(GL_TEXTURE_2D, 0);
	return tex;
}

int main(int argc, char** argv){
    SDL_Init(SDL_INIT_VIDEO);
	
	SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
	SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
	SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
	SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 16);
	SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
	
	if (SDL_SetVideoMode(scrWidth, scrHeight, 32, SDL_OPENGL) == 0){
		fprintf(stderr, "couldn't set mode %dx%d!\n", 640, 480);
		SDL_Quit();
		return -1;
	}
	
	SDL_ShowCursor(SDL_DISABLE);
    
	glClearColor(0.2, 0.2, 0.2, 0.2);
	glClearDepth(1.0);
	
	glEnable(GL_DEPTH_TEST);
	glEnable(GL_TEXTURE_2D);
	texture = genTexture();
	
	bool running = true;
	Uint32 lastTick = SDL_GetTicks();
	Uint32 curTick = lastTick;
	while (running){
		SDL_Event event;
		if (SDL_PollEvent(&event)){
			switch(event.type){
				case SDL_QUIT:
				    running = false;
				    break;
			};
		}
		
		curTick = SDL_GetTicks();
		float delta = float(curTick - lastTick)/1000.0f;
		lastTick = curTick;
		
		glMatrixMode(GL_PROJECTION);
		glLoadIdentity();
		gluOrtho2D(0, scrWidth, scrHeight/2.0f, -scrHeight/2.0f);
	
		glMatrixMode(GL_MODELVIEW);
		glLoadIdentity();
		glBindTexture(GL_TEXTURE_2D, texture);
		static float offset = 0;
		glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
		glColor3f(0, 1, 0);
		const int numSegments = 4;
		float segmentSize = 1.0f/(float)numSegments;
		float halfSegment = segmentSize * 0.5f;
		float maxValue = ((float)(numSegments) - 0.5f)*segmentSize;
		float minValue = 0.5f*segmentSize;
		
		offset += delta*segmentSize*0.5f;
		if (offset > 1.0f)
			offset = offset - 1.0f;
		float lineOffset = offset;
		float cursorPos = 0.5f;
		lineOffset -= halfSegment;
		if (lineOffset < 0){
			cursorPos = lineOffset*numSegments + 0.5f;
			lineOffset = 0;
		}
		
		if (lineOffset > (1.0f - segmentSize)){
			cursorPos = (lineOffset - (1.0f - segmentSize))* numSegments + 0.5f;
			lineOffset = (1.0f - segmentSize);
		}

		glColor3f(1, 1, 1);
		glBegin(GL_TRIANGLE_FAN);
		float s1 = lineOffset;
		float s2 = lineOffset + segmentSize;
		glTexCoord2f(s1, 0);
		glVertex2f(0, -scrHeight*0.5f);
		glTexCoord2f(s2, 0);
		glVertex2f(scrWidth, -scrHeight*0.5f);
		glTexCoord2f(s2, 1);
		glVertex2f(scrWidth, scrHeight*0.5f);
		glTexCoord2f(s1, 1);
		glVertex2f(0, scrHeight*0.5f);
		glEnd();
		glBindTexture(GL_TEXTURE_2D, 0);
		glColor3f(0, 1, 0);
		glBegin(GL_LINE_STRIP);
		glVertex2f(cursorPos*scrWidth, scrHeight*0.5f);
		glVertex2f(cursorPos*scrWidth, -scrHeight*0.5f);
		glEnd();
			
		glDisable(GL_DEPTH_TEST);
		
		glFlush();
		SDL_GL_SwapBuffers();		
	}

	glDeleteTextures(1, &texture);	
	
    SDL_Quit();
    return 0;
}