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Loading bitmap

Started by tnutty Mar 8, 2009 at 7:09 PM 20 replies 3.2k views

Original Post

Mar 8, 2009 at 7:09 PM

I have a bitmap function that load bit map as shown below. The problem is that this function does not stretch the whole star into a quads for which I use transparency in order to make it look like only the star is there and not the quad. Instead, each quad has 3 small star and a wried line underneath the star. So, the quads thats supposed to look like a single star because of glblend, does not. Can you see why this is doing so. The end result of my program should look like a spiral of stars constantly swirling around the center, but instead, it looks like 3 start for each star I load onto a quad. here is what my star looks like : http://img186.imageshack.us/my.php?image=mystar.jpg whereas you could imagine what it should look like here is my Bmploader()


#include<GL/glut.h>
#include<cstdio>
#include<windows.h>
#include"Global.h"

int texID = -1;

int loadBitmap(char * Filename)
{
	unsigned char *ucTexSize;
	RGBTRIPLE rgb;
	BITMAPFILEHEADER fHeader;
	BITMAPINFOHEADER iInfo;
	FILE* pFile = NULL;

	texID++;

	pFile = fopen(Filename,"rb");
	
	if(pFile == NULL)
	{
		MessageBox(NULL,L"ERROR: READING BITMAP FAILED!",L"LOADING BITMAP IMAGE",MB_OK);
		fclose(pFile);
		return -1;
	}

	fread(&fHeader,sizeof(fHeader),1,pFile);
	fseek(pFile,sizeof(fHeader),SEEK_SET);
	fread(&iInfo,sizeof(iInfo),1,pFile);


	ucTexSize = (byte *) malloc(iInfo.biWidth * iInfo.biHeight * 4);
	memset(ucTexSize,0,iInfo.biWidth*iInfo.biHeight*4);

	for(int i=0,j=0;i<iInfo.biHeight*iInfo.biWidth;i++)
	{
		fread(&rgb,sizeof(rgb),1,pFile);
		
		ucTexSize[j+0] = rgb.rgbtRed;
		ucTexSize[j+1] = rgb.rgbtGreen;
		ucTexSize[j+2] = rgb.rgbtBlue;
		ucTexSize[j+3] = 255;
		j+=4;
	}

	
	fclose(pFile);

	glBindTexture(GL_TEXTURE_2D,texID);
	glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_GEN_S,GL_REPEAT);
	glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_GEN_T,GL_REPEAT);
	glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
	glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
	glTexEnvf(GL_TEXTURE_2D,GL_TEXTURE_ENV_MODE,GL_REPLACE);
	
	glTexImage2D(GL_TEXTURE_2D,0,4,iInfo.biWidth,iInfo.biHeight,0,GL_RGBA,GL_UNSIGNED_BYTE,ucTexSize);
	gluBuild2DMipmaps(GL_TEXTURE_2D,4,iInfo.biWidth,iInfo.biHeight,GL_RGBA,GL_UNSIGNED_BYTE,ucTexSize);


	free(ucTexSize);

	return(texID);
}

mobeen

Mar 8, 2009 at 9:59 PM

Hi,
This is related to how bitmaps are stored in memory. If you have a power of two bitmap, it fits nicely into the memory but if there is a non power of two bitmap, every single scan line (stride) will be padded with zeros so that it can fit nicely into the memory. You need to calculate the pitch and then use that information while you load the bitmap.

For power of two bitmaps,
stride =bitmap width
but for non power of two bitmaps, it's
stride =bitmap width + (zero padding width);

I think you might be assuming that the total number of bytes in bitmap are
totalBytes =bmWidth*bmHeight*bmBytesPerPixel;
this is ok only for power of two images and will fail for non power of two images.
The solution is to use the bmWidthBytes field of the BITMAP struct that contains the whole stride width. So the total bytes are
totalBytes = bmHeight*bmWidthBytes;

Here is a simple win32 based bmp loader code see if this helps.

void LoadBMP(const char* filename, GLubyte** imgPtr, int& w, int& h ){   HBITMAP hbmp;    BITMAP	bmp;    hbmp=(HBITMAP)LoadImage(GetModuleHandle(NULL), filename, IMAGE_BITMAP, 0, 0, LR_CREATEDIBSECTION | LR_LOADFROMFILE );   GetObject(hbmp, sizeof(bmp), &bmp);  				   		   w = bmp.bmWidth;   h = bmp.bmHeight;    int size = bmp.bmHeight*bmp.bmWidthBytes;   *imgPtr=new GLubyte[size];   memcpy(*imgPtr,bmp.bmBits,size);	   DeleteObject(hbmp);	}

Hope this helps,
Mobeen

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