Opengl Pixel Co-ordinate manipulation - c++

Question

Ive been looking through the internet for a while now trying to find out if i can DIRECTLY manipulate Pixels like those that makes up the triangle in a mesh not vertex co-ordinates.
When a mesh is being formed in OpenGL the vertex co-ordinates that forms independent triangles are each filled with Pixels that gives it color.
Those pixels are what im trying to manipulate. So far in every Tutorial all i'm seeing is how to alter vertex coords even in the Fragment shader parts of Glsl tutorials i'm not finding anything on the Pixels directly. I'm being shown Texture and Vertex co-ordinates no direct Pixel manipulation.
So far what i know happens is each vertex is assigned some color value and all the Pixel processes get done during execution and you see the results.
So can Pixels be directly altered in OpenGl for each Triangle or What would u guys recommend? Cuz i've heard it might be possible in OpenCV but thats stuff is about Textures

Answer 1

If I get it right you have high poly mesh and want to simplify it by creating normal map for smaller poly count faces ...
Never done this but I would attack this problem like this:
create UV mapping of high poly mesh
create low poly mesh
so you need to merge smaller adjacent faces into bigger ones. Merging only faces that are not too angled to starting face (abs dot between normals is smaller than threshold)... You also need to remember original mesh of merged face.
for each merged face render normal
so render the merged face original polygon to texture but use UV as 2D vertex coordinates and output actual triangle normal as color
This will copy the normals into normal map at the correct position. Do not use any depth buffering blending lighting or whatever. Also the 2D view must be scaled and translated so the UV mapping will cover your texture (no perspective) Do not forget that the normal map (if RGB float used) is clamped so you should first normalize the normal and then convert to range <0,1> for example:
n = 0.5 * (vec3(1.0,1.0,1.0) + normalize(n));
read back the rendered texture
now it should hold the whole normal map. In case you do not have Render to texture available (older Intel HD) you can render to screen instead and then just use glReadPixels.
As you want to save this to image here a small VCL example of saving to 24 bit bmp:
//---------------------------------------------------------------------------
void screenshot(int xs,int ys) // xs,ys is GL screen resolution
{
// just in case your environment does not know basic programing datatypes
typedef unsigned __int8 BYTE;
typedef unsigned __int16 WORD;
typedef unsigned __int32 DWORD;
xs&=0xFFFFFFFC; // crop down resolution to be divisible by 4
ys&=0xFFFFFFFC; // in order make glReadPixel not crashing on some implementations
BYTE *dat,zero[4]={0,0,0,0};
int hnd,x,y,a,align,xs3=3*xs;
// allocate memory for pixel data
dat=new BYTE[xs3*ys];
if (dat==NULL) return;
// copy GL screen to dat
glReadPixels(0,0,xs,ys,GL_BGR,GL_UNSIGNED_BYTE,dat);
glFinish();
// BMP header structure
#pragma pack(push,1)
struct _hdr
{
char ID[2];
DWORD size;
WORD reserved1[2];
DWORD offset;
DWORD reserved2;
DWORD width,height;
WORD planes;
WORD bits;
DWORD compression;
DWORD imagesize;
DWORD xresolution,yresolution;
DWORD ncolors;
DWORD importantcolors;
} hdr;
#pragma pack(pop)
// BMP header extracted from uncompressed 24 bit BMP
const BYTE bmp24[sizeof(hdr)]={0x42,0x4D,0xE6,0x71,0xB,0x0,0x0,0x0,0x0,0x0,0x36,0x0,0x0,0x0,0x28,0x0,0x0,0x0,0xF4,0x1,0x0,0x0,0xF4,0x1,0x0,0x0,0x1,0x0,0x18,0x0,0x0,0x0,0x0,0x0,0xB0,0x71,0xB,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0,0x0};
// init hdr with 24 bit BMP header
for (x=0;x<sizeof(hdr);x++) ((BYTE*)(&hdr))[x]=bmp24[x];
// update hdr stuf with our image properties
align=0; // (4-(xs3&3))&3;
hdr.size=sizeof(hdr)+(ys*(xs3+align));
hdr.width=xs;
hdr.height=ys;
hdr.imagesize=ys*xs3;
// save BMP file (using VCL file functions exchange them with whatever you got)
hnd=FileCreate("screenshot.bmp"); // create screenshot image file (binary)
if (hnd!=-1) // if file created
{
FileWrite(hnd,&hdr,sizeof(hdr));// write bmp header
for (a=0,y=0;y<ys;y++,a+=xs3) // loop through all scan lines
{
FileWrite(hnd,&dat[a],xs3); // write scan line pixel data
if (align) // write scan line align zeropad if needed
FileWrite(hnd,zero,align);
}
FileClose(hnd); // close file
}
// cleanup before exit
delete[] dat; // release dat
}
//---------------------------------------------------------------------------
The only thing used from VCL are binary file access routines so just swap them with what you have at disposal. Now you can open this bmp in whatever image software and convert to whatever format you want like png... without the need to encode it yourself.
The bmp header structure was taken from this QA:
Importing BMP file in Turboc++ issue:BMP file is not being displayed properly in the output screen
Also beware of using char/int instead of BYTE/WORD/DWORD it usually leads to data corruption for tasks like this if you do not know what you doing...
You can do the same with color if the mesh is textured ... That way the normal map and color map would have the same UV mapping even if the original mesh uses more than single texture ...