# OpenGL Concept Clarification - Transformations

## Recommended Posts

Cirus    122
Hello All, I am newto OpenGl and have been refering to The Red Book... for a few days. The book says that you can either view al transformations by taking coordinate axes as fixed or in terms of local coordinate system( tied to a rendered object). Question 1. : the glLoadIdentity maps a window to a [-1,+1] scale ( for each X,Y and Z). Now if I perfrom following glMatrixMode(GL_MODELVIEW); glLoadIdentity(); glPushMatrix(); glRotatef(90.0 , 0.0 ,1.0 , 0.0) ; //Rotation around Y axis , anticlockwise, Will my -X (negative X axis) take on the position of +Z axis and + Z axis replaces +X axis and +X axis replace -Z axis ??? Question 2: My second question reuires the answer of Ques 1 because I am aiming to construct an external control mesh , housing the object. The object I am taking is a cube such that the origin of the axes lie at the centroid of the cube. I am planning to contruct the external control mesh by asking user to mark points near to object , so that the points becomes lines( when I rotate the object). Problem is: At default Y axis is up-down, X is sideways and Z is front-back. When I do anticlockwise rotation across Z and mark point (by calling glVertex3f), the line so drawn is not horizontal but vertical . Any suggestions on my queries? I will post the code also , but need to get a suggestion first. Thanks in advance.

##### Share on other sites
mhamlin    520
Quote:
 Original post by CirusQuestion 1. : the glLoadIdentity maps a window to a [-1,+1] scale ( for each X,Y and Z). Now if I perfrom following glMatrixMode(GL_MODELVIEW);glLoadIdentity();glPushMatrix();glRotatef(90.0 , 0.0 ,1.0 , 0.0) ; //Rotation around Y axis , anticlockwise, Will my -X (negative X axis) take on the position of +Z axis and + Z axis replaces +X axis and +X axis replace -Z axis ???

Yes; you should really do the math on some paper to get a better feel for what's actually happening, though.

glRotatef(90, 0, 1, 0) puts this matrix
[ 0  0 1 0 ][ 0  1 0 0 ][ -1 0 0 0 ][ 0  0 0 1 ]

onto the modelview matrix stack (it is postmultiplied by the current contents of the modelview matrix, which happens to be the identity matrix now). Try multiplying (column) vectors with this matrix to get a good feel.

Quote:
 Original post by CirusQuestion 2: My second question reuires the answer of Ques 1 because I am aiming to construct an external control mesh , housing the object. The object I am taking is a cube such that the origin of the axes lie at the centroid of the cube. I am planning to contruct the external control mesh by asking user to mark points near to object , so that the points becomes lines( when I rotate the object). Problem is: At default Y axis is up-down, X is sideways and Z is front-back.When I do anticlockwise rotation across Z and mark point (by calling glVertex3f), the line so drawn is not horizontal but vertical .

I'm not really sure what you mean by this, perhaps you can rephrase your issue?

##### Share on other sites
dmatter    4844
Quote:
 Original post by CirusProblem is: At default Y axis is up-down, X is sideways and Z is front-back.When I do anticlockwise rotation across Z and mark point (by calling glVertex3f), the line so drawn is not horizontal but vertical .

Im not sure what your saying here but im going to take a guess..

When you say rotation 'across Z' are you doing:
glRotatef(angle, 0.0, 0.0, 1.0);

If this is the case then your actually rotating around the Z-axis. In this situation if you rotate and plot a point then rotate a little bit more and plot another point (possibly to draw a line?) then it would appear that the points are moving vertically (actually vertically and horizontally), in actual fact if you continued like this you would end up drawing a circle.

To plot points across the Z axis you need to rotate around the Y-axis (as you did in Q1), but bear in mind the points would also be plotted across the X-axis as well as the Z-axis.

Im not sure if that helps at all, if not then re-phrase the question.

Happy Coding

##### Share on other sites
Cirus    122
Regarding Qeus 2:

I am given a task to construct an external contrl mesh around my object.

How do I do this? :
I take the object , and mouse click near to its edges and expect a point there.
This point is not a point but actually a line ( Imagine a line when placed in front can be viewed as a point ) .

To be precise , the line points in +Z direction that is out of computer screen.

Now , after this , I am rotating my object ( coordinate axes) in anti-clock wise Y or X direction .

After performing a rotation the point (the tip of the line ) will be viewed as a line.

Problem:
If a rotate the object around +Z axis and then mouse click I am getting the point at some different position on the screen other than the expected position.

--------------------------------------------------------------------------
void drawPoint()
{
int i =0 ;

glEnable(GL_DEPTH_TEST);
// Set the material color to follow the current color

glMatrixMode(GL_MODELVIEW);

glPushMatrix();
glRotatef(fAngle_Y , 0.0 , 1.0 , 0.0); //Move left
glRotatef(fAngle_X , 1.0 , 0.0 , 0.0); //Move above
glRotatef(fAngle_Z , 0.0 , 0.0 , 1.0); //Move above

glBegin(GL_POINTS);

while( i != iNumOfClicks)
{
glColor3f(1.0 , 0.0 , 1.0);
glVertex3f(pfArray[i] + 0.25, pfArray[i + 1], pfArray[i + 2] );
glVertex3f(pfArray[i] + 0.25, pfArray[i + 1], pfArray[i + 2] - 0.5 );

++i;

}
glEnd();
glPopMatrix();
i = 0; //Reset counter.

if(bFlag_LineDraw)
{
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glRotatef(fAngle_Y , 0.0 , 1.0 , 0.0); //Move left
glRotatef(fAngle_X , 1.0 , 0.0 , 0.0); //Move above
glRotatef(fAngle_Z , 0.0 , 0.0 , 1.0); //Move above
glBegin(GL_LINES);
while( i != iNumOfClicks)
{
glColor3f(1.0 , 0.0 , 1.0);
glVertex3f(pfArray[i] + 0.25, pfArray[i + 1], pfArray[i + 2] );
glVertex3f(pfArray[i] + 0.25, pfArray[i + 1], pfArray[i + 2] - 0.5 );
i++;
}
glEnd();
glPopMatrix();

bFlag_LineDraw = FALSE;
}
-----------------------------------------------------------------------------

This is the function that is converting viewport coordinates to world coordinates {-1,+1] range
----------------------------------------------------------------------------
void MarkPointOnScreen(int iX , int iY, int iWinHt , int iWinWidth, float *pfArray)
{
//Draw the point on screen.
float fx = 0.0f , fy = 0.0f;
if( iX > (iWinWidth/2) )
{
fx = ( (float)iX - (float)iWinWidth/2 ) / (float)(iWinWidth/2);
}
else if( iX == iWinWidth/2)
fx = 0.0f;
else
{

fx = -1 * ( (float)iWinWidth/2 - (float)iX ) / (float)(iWinWidth/2);

}

if(iY > iWinHt/2)
{

fy = -1 * ( (float)iY - (float)(iWinHt/2) ) / (float)(iWinHt/2);
}
else if(iY == iWinHt/2)
fy = 0.0f;
else
{

fy = ( (float)(iWinHt/2) - (float)iY ) / (float)(iWinHt/2);

}

pfArray[iEle++] = fx;
pfArray[iEle++] = fy;
pfArray[iEle++] = 0.0f;

printf("\niX = %d and iY = %d , fx = %f and fy = %f , Clicks = %d \n", iX,iY,fx ,fy, iNumOfClicks);
}

## Create an account

Register a new account

• ### Similar Content

• I assumed that if a shader is computationally expensive then the execution is just slower. But running the following GLSL FS instead just crashes
void main() { float x = 0; float y = 0; int sum = 0; for (float x = 0; x < 10; x += 0.00005) { for (float y = 0; y < 10; y += 0.00005) { sum++; } } fragColor = vec4(1, 1, 1 , 1.0); } with unhandled exception in nvoglv32.dll. Are there any hard limits on the number of steps/time that a shader can take before it is shut down? I was thinking about implementing some time intensive computation in shaders where it would take on the order of seconds to compute a frame, is that possible? Thanks.

• There are studios selling applications which is just copying any 3Dgraphic content and regenerating into another new window. especially for CAVE Virtual reality experience. so that the user opens REvite or CAD or any other 3D applications and opens a model. then when the user selects the rendered window the VR application copies the 3D model information from the OpenGL window.
I got the clue that the VR application replaces the windows opengl32.dll file. how this is possible ... how can we copy the 3d content from the current OpenGL window.

Thanks
• By cebugdev
hi all,

i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
thank you, and looking forward to positive replies.

• I have a few beginner questions about tesselation that I really have no clue.
The opengl wiki doesn't seem to talk anything about the details.

What is the relationship between TCS layout out and TES layout in?
How does the tesselator know how control points are organized?
e.g. If TES input requests triangles, but TCS can output N vertices.
What happens in this case?
http://www.informit.com/articles/article.aspx?p=2120983
the isoline example TCS out=4, but TES in=isoline.
And gl_TessCoord is only a single one.
So which ones are the control points?
How are tesselator building primitives?
• By Orella
I've been developing a 2D Engine using SFML + ImGui.
Here you can see an image
The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine.
I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui.
3D Editor preview
But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
If you can provide code will be better. And if you want me to provide any specific code tell me.
Thanks!

• 11
• 9
• 35
• 13
• 39