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OpenGL How to use pixel coordinate in OpenGL?

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I use glVertex2f and the displayed position not be what I expected as from upper-left corner of screen, but the center of screen and the value must between 0.0 to 1.0(to be inside a window)! My source code:
/**************************
 * Includes
 *
 **************************/

#include <windows.h>
#include <gl/gl.h>

unsigned long image[16] = {
	0xFF0000FF, 0xFF0000FF, 0xFF0000FF, 0xFF0000FF,
	0xFF0000FF, 0xFF0000FF, 0xFF0000FF, 0xFF0000FF,
	0xFF0000FF, 0xFF0000FF, 0xFF0000FF, 0xFF0000FF,
	0xFF0000FF, 0xFF0000FF, 0xFF0000FF, 0xFF0000FF,
};
unsigned int texobj[1];
/**************************
 * Function Declarations
 *
 **************************/

LRESULT CALLBACK WndProc (HWND hWnd, UINT message,
WPARAM wParam, LPARAM lParam);
void EnableOpenGL (HWND hWnd, HDC *hDC, HGLRC *hRC);
void DisableOpenGL (HWND hWnd, HDC hDC, HGLRC hRC);


/**************************
 * WinMain
 *
 **************************/

int WINAPI WinMain (HINSTANCE hInstance,
                    HINSTANCE hPrevInstance,
                    LPSTR lpCmdLine,
                    int iCmdShow)
{
    WNDCLASS wc;
    HWND hWnd;
    HDC hDC;
    HGLRC hRC;        
    MSG msg;
    BOOL bQuit = FALSE;

    /* register window class */
    wc.style = CS_OWNDC;
    wc.lpfnWndProc = WndProc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = hInstance;
    wc.hIcon = LoadIcon (NULL, IDI_APPLICATION);
    wc.hCursor = LoadCursor (NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH) GetStockObject (BLACK_BRUSH);
    wc.lpszMenuName = NULL;
    wc.lpszClassName = "GLSample";
    RegisterClass (&wc);

    /* create main window */
    hWnd = CreateWindow (
      "GLSample", "OpenGL Sample", 
      WS_CAPTION | WS_POPUPWINDOW | WS_VISIBLE,
      0, 0, 256, 256,
      NULL, NULL, hInstance, NULL);

    /* enable OpenGL for the window */
    EnableOpenGL (hWnd, &hDC, &hRC);
	glEnable(GL_BLEND);
	glEnable(GL_TEXTURE_2D);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
	glGenTextures(1, texobj);
	glBindTexture(GL_TEXTURE_2D, texobj[0]);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 4, 4, 0, GL_RGBA, GL_UNSIGNED_BYTE, image);

    /* program main loop */
    while (!bQuit)
    {
        /* check for messages */
        if (PeekMessage (&msg, NULL, 0, 0, PM_REMOVE))
        {
            /* handle or dispatch messages */
            if (msg.message == WM_QUIT)
            {
                bQuit = TRUE;
            }
            else
            {
                TranslateMessage (&msg);
                DispatchMessage (&msg);
            }
        }
        else
        {
            /* OpenGL animation code goes here */

            glClearColor (0.0f, 0.0f, 0.0f, 0.0f);
            glClear (GL_COLOR_BUFFER_BIT);

            glBegin (GL_QUADS);
            glTexCoord2f(0.0, 0.0);glColor4f(1.0f, 1.0f, 1.0f, 1.0f);glVertex2f(0.0f, 0.0f);
            glTexCoord2f(1.0, 0.0);glColor4f(1.0f, 1.0f, 1.0f, 1.0f);glVertex2f(0.5f, 0.0f);
            glTexCoord2f(1.0, 1.0);glColor4f(1.0f, 1.0f, 1.0f, 1.0f);glVertex2f(0.5f, 0.5f);
			glTexCoord2f(0.0, 1.0);glColor4f(1.0f, 1.0f, 1.0f, 1.0f);glVertex2f(0.0f, 0.5f);
            glEnd ();

            SwapBuffers (hDC);

            Sleep (1);
        }
    }

    /* shutdown OpenGL */
    DisableOpenGL (hWnd, hDC, hRC);

    /* destroy the window explicitly */
    DestroyWindow (hWnd);

    return msg.wParam;
}


/********************
 * Window Procedure
 *
 ********************/

LRESULT CALLBACK WndProc (HWND hWnd, UINT message,
                          WPARAM wParam, LPARAM lParam)
{

    switch (message)
    {
    case WM_CREATE:
        return 0;
    case WM_CLOSE:
        PostQuitMessage (0);
        return 0;

    case WM_DESTROY:
        return 0;

    case WM_KEYDOWN:
        switch (wParam)
        {
        case VK_ESCAPE:
            PostQuitMessage(0);
            return 0;
        }
        return 0;

    default:
        return DefWindowProc (hWnd, message, wParam, lParam);
    }
}


/*******************
 * Enable OpenGL
 *
 *******************/

void EnableOpenGL (HWND hWnd, HDC *hDC, HGLRC *hRC)
{
    PIXELFORMATDESCRIPTOR pfd;
    int iFormat;

    /* get the device context (DC) */
    *hDC = GetDC (hWnd);

    /* set the pixel format for the DC */
    ZeroMemory (&pfd, sizeof (pfd));
    pfd.nSize = sizeof (pfd);
    pfd.nVersion = 1;
    pfd.dwFlags = PFD_DRAW_TO_WINDOW | 
      PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;
    pfd.iPixelType = PFD_TYPE_RGBA;
    pfd.cColorBits = 24;
    pfd.cDepthBits = 16;
    pfd.iLayerType = PFD_MAIN_PLANE;
    iFormat = ChoosePixelFormat (*hDC, &pfd);
    SetPixelFormat (*hDC, iFormat, &pfd);

    /* create and enable the render context (RC) */
    *hRC = wglCreateContext( *hDC );
    wglMakeCurrent( *hDC, *hRC );

}


/******************
 * Disable OpenGL
 *
 ******************/

void DisableOpenGL (HWND hWnd, HDC hDC, HGLRC hRC)
{
    wglMakeCurrent (NULL, NULL);
    wglDeleteContext (hRC);
    ReleaseDC (hWnd, hDC);
}

So, can I use pixel coordinate(as in Direct3D) in OpenGL?

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Guest Anonymous Poster
glOrtho/gluOrtho2D

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Quote:
Original post by Anonymous Poster
glOrtho/gluOrtho2D


To expand on this:

glOrtho( -0.5, width-0.5, -0.5, height-0.5, -1, 1); will put 0,0 at the bottom left.
glOrtho( -0.5, width-0.5, height-0.5, -0.5, -1, 1); will put 0,0 at the top left.

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I have put it into my code, but it become nothing displayed.
Have I missed other stuff in my code?

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Rick Appleton : this puts 0 0 in the middle. glOrtho is not a translation.

Opengl's 0,0 is bottom left.

So with glOrtho :

glOrtho( left, right, bottom, top, near, far )


this means you have left = 0, right = 1, bottom = 0, top = 1, near = -1, far = -1

(0,1)------(1,1)

(0,0)-------(1,0)

if you want to have your triangle at the upper left corner, you have to change your y-coord to match the height.

glVertex2f(0.0f, 1.0f);
glVertex2f(0.5f, 1.0f);
glVertex2f(0.5f, 0.5f);
glVertex2f(0.0f, 0.5f);

you can also increase the size .
left = 0, right = 1024, bottom = 0, top = 768, near = -1, far = 1

the means you are now working with a screen of size

(0,768)------(1024,768)

(0,0)-------(1024,0)

Generically,
left = 0, right = width, bottom= 0, top = height, near = -1, far = 1

To have at the upper left corner

glVertex2f(0.0f, height - 0.0f);
glVertex2f(0.5f, height - 0.0f);
glVertex2f(0.5f, height - 0.5f);
glVertex2f(0.0f, height - 0.5f);

Of course, if you pick a large width and height, make the sides of your square bigger than 0.5f or it might become too small.

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Guest Anonymous Poster

Anyway use this function,

Method Draw2D()

drawMode = DrawMode_2D
glViewport 0, 0, vWidth,vHeight
glMatrixMode gl_projection
glLoadIdentity()
gluOrtho2D(0.0, vWidth, 0.0, vHeight )
glMatrixMode gl_modelview
glLoadIdentity
glDisable(gl_depth_test)
glDisable(gl_cull_face)
glDisable(GL_LIGHTING)

End Method

to enable 2d projection. now you can specify vertices in pixel space.

but it's from bottom to top instead of traditional top to bottom.

so you have to do this,

int y = 20
int useY = windowHeight-y

glVertex2f 20,useY

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Quote:
Original post by Gorg
Rick Appleton : this puts 0 0 in the middle. glOrtho is not a translation.

Opengl's 0,0 is bottom left.

So with glOrtho :

glOrtho( left, right, bottom, top, near, far )


this means you have left = 0, right = 1, bottom = 0, top = 1, near = -1, far = -1

(0,1)------(1,1)

(0,0)-------(1,0)

if you want to have your triangle at the upper left corner, you have to change your y-coord to match the height.

glVertex2f(0.0f, 1.0f);
glVertex2f(0.5f, 1.0f);
glVertex2f(0.5f, 0.5f);
glVertex2f(0.0f, 0.5f);

you can also increase the size .
left = 0, right = 1024, bottom = 0, top = 768, near = -1, far = 1

the means you are now working with a screen of size

(0,768)------(1024,768)

(0,0)-------(1024,0)

Generically,
left = 0, right = width, bottom= 0, top = height, near = -1, far = 1

To have at the upper left corner

glVertex2f(0.0f, height - 0.0f);
glVertex2f(0.5f, height - 0.0f);
glVertex2f(0.5f, height - 0.5f);
glVertex2f(0.0f, height - 0.5f);

Of course, if you pick a large width and height, make the sides of your square bigger than 0.5f or it might become too small.


Which is exactly what I said. If you enter 1.0 for width and height, then you get 0,0 in the center. If you enter width and height (1024,768) you'll get what he wants. The 0.5 subtracted is done to get the pixel centers correct. If you don't do that then a point plotted at 0,0, will be off-screen for three-quarters. And if you set y-min=height y-max=0, you can then draw things as if top left was 0,0, so there's no need to take that into account for each call as you do.

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Guest Anonymous Poster
you can use opengl without glOrtho and any matrix process function if you just do very simple 2d stuff, I think it will much faster!

say, if your window size is 256x256, then size of one pixel will be:
x-axis: 1.0/(256.0/2.0);
y-axis: 1.0/(256.0/2.0);
(formula: 1.0/((height or width of your window)/2.0);

if you then position it at (3,5), the final value will be:(note its from bottom-left corner!)
x-value: 3*1.0/(256.0/2.0)-1.0;
y-value: 5*1.0/(256.0/2.0)-1.0;
(formula: (x or y pos)*(size of one pixel in x or y)-1.0;

you can alway precompute it. I think it will still be faster than matrix process even if you do it at run time.

GOOD LUCK!

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You still haven't tell me what change shall be add to my code. I wanna do some 2D stuff and I have posted a thread about will I get more speed if I don't use depth and any matrix-relative OpenGL API but no one give me a idea.
Quote:
Original post by Anonymous Poster
you can use opengl without glOrtho and any matrix process function if you just do very simple 2d stuff, I think it will much faster!

say, if your window size is 256x256, then size of one pixel will be:
x-axis: 1.0/(256.0/2.0);
y-axis: 1.0/(256.0/2.0);
(formula: 1.0/((height or width of your window)/2.0);

if you then position it at (3,5), the final value will be:(note its from bottom-left corner!)
x-value: 3*1.0/(256.0/2.0)-1.0;
y-value: 5*1.0/(256.0/2.0)-1.0;
(formula: (x or y pos)*(size of one pixel in x or y)-1.0;

you can alway precompute it. I think it will still be faster than matrix process even if you do it at run time.

GOOD LUCK!

Thanks! But it's just your *think*, it is really a pity... It will be meaningless if it is not faster, and I do not know whether it will cause something unexpected.

I want more info about minimally setting OpenGL without use depth and matrix and it's speed.

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Quote:
I wanna do some 2D stuff and I have posted a thread about will I get more speed if I don't use depth and any matrix-relative OpenGL API but no one give me a idea.

For 2D drawing, you only need to set the model-view and projection matrices once (well, whenever the size of the viewport changes), so there is no performance issue.

glMatrixMode(GL_PROJECTION);
gluOrtho2D(0, 0, width, height);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

Quote:
I want more info about minimally setting OpenGL without use depth and matrix and it's speed.

All vertices you send will be transformed by the model-view and projection matrices. You do want to minimize the number of changes you make to the matrices per frame, but in your case that's 0 and you can't get much faster than that.

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