# OpenGL Texture Tiling How-to

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Greetings everybody, I am working on the next version of my graphics library, and I am trying to solve the problem of using an OpenGL texture map for an image surface. Due to the resrictions on size of the texture, I figured I could solve this by breaking up the surface into smaller textures. But that just brings up more problems, such as how to figure out UV coordinates and screen coordinates. This is my blitting function, and where I think the problem is
[source ]void hxSurface::Blit(hxRect sRect, hxRect dRect)
{
float u1, u2, v1, v2;	//texture coordinates
float sx, sy, sw, sh;	//screen coordinates

int l, r, t, b;			//four edges of the image

l = sRect.x / this->chunk_width;
r = (sRect.x + sRect.w) / this->chunk_width;

t = sRect.y / this->chunk_height;
b = (sRect.y + sRect.h) / this->chunk_height;

//screen coordinate variables
float x_start = FL(dRect.x);
float y_start = FL(dRect.y);

float w_inc = FL(dRect.w)/FL(sRect.w) * FL(this->chunk_width);
float h_inc = FL(dRect.w)/FL(sRect.w) * FL(this->chunk_height);

float xOff, yOff;	//offsets for stuff

sx = x_start;
sx = y_start;

xOff = w_inc;
yOff = h_inc;

for (int i = t; i <= b; i += 1)
{
for(int j = l; j <= r; j += 1)
{
u1 = 0.0f;
v1 = 0.0f;

u2 = 1.0f;
v2 = 1.0f;

//do adjustments for UV coordinates
if(j == l)
u1 = FL(l % this->chunk_width) / FL(this->chunk_width);

if(j == r)
u2 = FL(r % this->chunk_width) / FL(this->chunk_width);

if(i == t)
v1 = FL(t % this->chunk_height) / FL(this->chunk_height);

if(i == b)
v2 = FL(l % this->chunk_height) / FL(this->chunk_height);

//do adjustments for screen coordinates
sw = w_inc * (u2 - u1);
sh = h_inc * (v2 - v1);

//blit object to the screen;
glBindTexture(GL_TEXTURE_2D, this->texList[(this->y_chunk * i)+j]);

glTexCoord2f(u1, v1);	glVertex2f(sx, sy);
glTexCoord2f(u1, v2);	glVertex2f(sx, sy);
glTexCoord2f(u2, v2);	glVertex2f(sx + sw, sy + sh);
glTexCoord2f(u2, v1);	glVertex2f(sx + sw, sy);

glEnd();

sx += sw;

//this doesn't look right, does it?
if(j == r)
{
sy += sh;
sx = x_start;
}
}
}

return;
}


FL is a macro that casts something as a float. Is there anything blatently obvious that you see with the tiling? Should I be looking at another method instead?

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Just at first glance, should this line:

if(i == b) v2 = FL(l % this->chunk_height) / FL(this->chunk_height);

if(i == b) v2 = FL(b % this->chunk_height) / FL(this->chunk_height);

You're using l instead of b.

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I applied that change, and still nothing appeared.
For further refrence, here is some more of the code.

hxSurface constructor
hxSurface::hxSurface(uint w, uint h){	//Step 1 break image into the smallest ammount of chunks possible	//Until I write a better routine, 64x64 will have to do...	uint wChunkSize = 64;		//64 by 64 is going to be the smallest	uint hChunkSize = 64;	int wct;					//number of blocks in each direction	int hct;    if(w > 64)					//Determine the width and height of each of the chunks		wChunkSize = 128;	if(w > 128)		wChunkSize = 256;	if(w > 256)		wChunkSize = 512;		if(h > 64)		hChunkSize = 128;	if(h > 128)		hChunkSize = 256;	if(h > 256)		hChunkSize = 512;	wct = w / wChunkSize;	hct = h / hChunkSize;	if(w % wChunkSize)			//take care of any overlap		wct += 1;	if(h % hChunkSize)		hct += 1;	//Step 2, allocate some memory for all the mini surfaces	surfaceChunk **list;	uint *textureList;	list = (surfaceChunk **)malloc(sizeof(surfaceChunk)*wct*hct);	//Step 3, create a unique texture ID for each of those surfaces	textureList = (uint *)malloc(sizeof(uint)*wct*hct);	glGenTextures((wct*hct),textureList);	//Step 4, allocate memory for the chunks	for(int i = 0; i < hct; i += 1)	{		for(int j = 0; j < wct; j += 1)		{			list[(i * wct)+ j] = CreateChunk(wChunkSize, hChunkSize, textureList[(i*wct) + j]);		}	}	//Step 5, share the pointers with the rest of the class	this->total_width = w;	this->total_height = h;	this->chunk_width = wChunkSize;	this->chunk_height = hChunkSize;	this->x_chunk = wct;	this->y_chunk = hct;	this->numChunks = wct * hct;	this->chunks = list;	this->texList = textureList;}

here is the write pixel function:
void hxSurface::WritePixel(int x, int y, hxColor color){	surfaceChunk *workChunk;	//temporary work chunk container	int blockx;	//which block?	int blocky;	int offx;	//the actual x,y coordinates of the block	int offy;	blockx = (x / this->chunk_width);	blocky = (y / this->chunk_height);	offx = (x % this->chunk_width);	offy = (y % this->chunk_height);	workChunk = this->chunks[(blocky * this->x_chunk)+blockx];	WritePix(workChunk, offx, offy, color);	return;}

I just can't seem to narrow down where the problem is. Thanks to anybody who atleast reads this.

edit: here is the refresh surface function
void hxSurface::Update(void){	surfaceChunk *work;	for(int i = 0; i < this->numChunks; i += 1)	{		work = this->chunks;		glBindTexture(GL_TEXTURE_2D, work->texID);		//setup the texturing parameters		glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);			glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);		glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, work->w, work->h , 0, GL_RGBA, GL_UNSIGNED_BYTE, work->data);	}	return;}

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I'm prety sure that the UV coordinates are done alright, but it seems to be the screen coordinates that I am having problems generating.

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Anybody anybody at all?

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Have you tried stepping through the code and seeing what numbers are comming out right and which ones are not?

Set up some simple test cases and run them through the code and follow along. Veryify all the numbers as you go.

-=[ Megahertz ]=-

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Quote:
 Original post by MegahertzHave you tried stepping through the code and seeing what numbers are comming out right and which ones are not?Set up some simple test cases and run them through the code and follow along. Veryify all the numbers as you go.-=[ Megahertz ]=-

I'm doing this now, and for some reason, it's all comming back as null.

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"it's all comming back as null."

That's not very descriptive =P

-=[ Megahertz ]=-

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