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Please explain this code too me.


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#1 cMADsc   Members   -  Reputation: 122

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Posted 27 July 2001 - 07:56 AM

Greetings, I am working with nehe's lesson 6 loading a raw image. Some of the code seems to baffle me. I can get it to work I just need to logic about a line. Please explain this line........ sz = (((3*refimg->w+3)>>2)<<2)*refimg->h;
Here is the source code..... =============================================================== typedef struct _RGBIMG { GLuint w; // Image's Width GLuint h; // Image's Height GLubyte* data; // Image's Data (Pixels) } RGBIMG; GLuint g_texid[TEXTURES_NUM]; // Our Textures' Id List (NEW) bool load_rgb_image(const char* file_name, int w, int h, RGBIMG* refimg) { GLuint sz; // Our Image's Data Field Length In Bytes FILE* file; // The Image's File On Disk long fsize; // File Size In Bytes GLubyte* p; // Helper Pointer // Update The Image's Fields refimg->w = (GLuint) w; refimg->h = (GLuint) h; sz = (((3*refimg->w+3)>>2)<<2)*refimg->h; refimg->data = new GLubyte [sz]; if (refimg->data == NULL) return false; // Open The File And Read The Pixels file = fopen(file_name , "rb"); if (!file) return false; fseek(file, 0L, SEEK_END); fsize = ftell(file); if (fsize != (long)sz) { fclose(file); return false; } fseek(file, 0L, SEEK_SET); p = refimg->data; while (fsize > 0) { fread(p, 1, 1, file); p++; fsize--; } fclose(file); return true; } ============================================================= sz = (((3*refimg->w+3)>>2)<<2)*refimg->h; This is how I see the line... 3 multiply by the width plus 3. then shift the bytes right(divide by 2) the shift the bytes left (multiply by 2) multiply by the height. Please clear up this line for me. Thanks in advance. ----------------------------- "There are ones that say they can and there are those who actually do." "...u can not learn programming in a class, you have to learn it on your own." Edited by - cMADsc on July 27, 2001 3:03:47 PM

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#2 Beer Hunter   Members   -  Reputation: 712

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Posted 27 July 2001 - 12:08 PM

It''s finding the required amount of memory to allocate for the bitmap. The bitmap is 24-bit, so that''s 3 bytes per pixel. Hence the width is multiplied by 3. But each scan line is DWORD aligned; the actual required width for each line has to be rounded up to the nearest multiple of 4.

If you add 3 to a number, integrally divide by 4, then multiply by 4, the result is the number rounded up to the nearest multiple of 4.

So, the width required for each line is: ((3*refimg->w+3)>>2)<<2

Then that''s multiplied by the height of the bitmap to find the total required size.




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