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OpenGL glLoadIdentity and glTranslatef

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Hello,

I have been playing OpenGL for a while, but still have issues with some fundamental concept, for example glLoadIdentity and glTranslatef.

Can someone please explain to me whats the difference between the two:

1)
glLoadIdentity();
glTranslatef(30.0f, 30.0f, 0);

GLshort vertexArray[8] = {100, 100, 200, 100, 100, 300, 200, 300};
glVertexPointer(2, GL_SHORT, 0, vertexArray);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

This will move the rectangle.

2)
glLoadIdentity();

GLshort vertexArray[8] = {100, 100, 200, 100, 100, 300, 200, 300};
glVertexPointer(2, GL_SHORT, 0, vertexArray);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

glTranslatef(30.0f, 30.0f, 0);

This won't move the rectangle, why???????????

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Ok now, I get it.

glTranslatef only affect what will be sent to Matrix stack, NOT what has already in the Matrix stack.

glLoadIdentity copy what you have in the Matrix stack, say a set of data, these data can be modified by glTranslatef, because these data will be sent to the Matrix stack.

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Quote:
Original post by jskywalker
Ok now, I get it.

glTranslatef only affect what will be sent to Matrix stack, NOT what has already in the Matrix stack.

glLoadIdentity copy what you have in the Matrix stack, say a set of data, these data can be modified by glTranslatef, because these data will be sent to the Matrix stack.


That's not exactly right --

glTranslatef DOES affect what is currently on the matrix stack. If you want to return to the state before you called glTranslatef, you should save that state by using glPushMatrix before calling glTranslatef. When you are ready to return to the previous state, you use glPopMatrix.

glLoadIdentity does not copy anything. It completely replaces what is on the top of the matrix stack with the identity matrix. Again, if you want to return to the matrix you had before calling glLoadIdentity, you need to follow the procedure above using glPushMatrix and glPopMatrix.

Changes to the matrix do not affect any rendering calls made before the change, only after.

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Quote:
Original post by smr
glLoadIdentity does not copy anything. It completely replaces what is on the top of the matrix stack with the identity matrix.


Only this part I am still confusing.

Here is my code, very simple:


// first rectangle : yellow one
GLshort vertexArray1[8] = {0, 0, 5, 0, 0, 20, 5, 20};
glColor4f(1.0f, 1.0f, 0.0f, 1.0f);
glVertexPointer(2, GL_SHORT, 0, vertexArray1);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);



glLoadIdentity();
glRotatef(30.0f, 1.0f, 1.0f, 0);



// second rectangle: red one
GLshort vertexArray[8] = {-40, -40, -30, -40, -40, -20, -30, -20};
glColor4f(1.0f, 0, 0, 1.0f);
glVertexPointer(2, GL_SHORT, 0, vertexArray);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);


The result is it draws two yellow rectangles, the second yellow one rotates 30 degree. From the result, I can only think glLoadIdentity() just copied the original matrix. Sorry I feel really hard to think in mathematical matrix level, I am not good at pure math :-(.

[Edited by - jskywalker on August 27, 2010 1:03:34 PM]

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OK, let me use the following diagram to ask 2 more questions regarding the matrix thing.

A triangle matrix:
_ _
| 7 8 9 |
| -6 -4 -7 | original matrix (m1)
|_-9 -4 5 _|

*
_ _
| 1 0 1 |
| 0 1 0 | identity matrix ( glLoadIdentity(); )
|_ 0 0 1 _|

=
_ _
| 7 8 9 |
| -6 -4 -7 | current matrix (m2)
|_-9 -4 5 _|


Q1.

The matrix stack contains a set of matrix, only the top matrix (which is the current matrix) can be modified by glTranslatef , rotate or scale , is that correct? What about other old matrices ? They have been processed or waiting for being processed ?

Q2.

Similar question, see the diagram, after glLoadIdentity(), m2 becomes the current matrix, the top one in the matrix stack, am I correct ? if so, then using glTranslatef can affect the current matrix (m2), but not m1, correct ? Since m1 and m2 are identical, you will see (say the triangle) will be drawn twice.

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Quote:
Original post by jskywalker
OK, let me use the following diagram to ask 2 more questions regarding the matrix thing.

A triangle matrix:
_ _
| 7 8 9 |
| -6 -4 -7 | original matrix (m1)
|_-9 -4 5 _|

*
_ _
| 1 0 1 |
| 0 1 0 | identity matrix ( glLoadIdentity(); )
|_ 0 0 1 _|

=
_ _
| 7 8 9 |
| -6 -4 -7 | current matrix (m2)
|_-9 -4 5 _|


Q1.

The matrix stack contains a set of matrix, only the top matrix (which is the current matrix) can be modified by glTranslatef , rotate or scale , is that correct? What about other old matrices ? They have been processed or waiting for being processed ?

Q2.

Similar question, see the diagram, after glLoadIdentity(), m2 becomes the current matrix, the top one in the matrix stack, am I correct ? if so, then using glTranslatef can affect the current matrix (m2), but not m1, correct ? Since m1 and m2 are identical, you will see (say the triangle) will be drawn twice.


A1.

Yes the matrix stack contains a set of matrices. Every time you call glPushMatrix(), a copy of the top matrix is pushed to the top of the stack. When you call glPopMatrix, the top matrix is removed from the stack and the matrix under it becomes the new top matrix. And yes, only the top matrix is modified by translate, rotate, scale AND glLoadIdentity. The other matrices are not at the top of the stack, so these calls have no effect on them.

A2.

The diagram is wrong. glLoadIdentity REPLACES the matrix at the top of the stack with the identity matrix (implied by the word "Load" in the function name). There is also a function called glLoadMatrix that allows you to replace the current matrix with an arbitrary one of your choosing. glRotate, Translate and Scale however will do multiplication. So m2 in your diagram would be the identity matrix. The rest of your question is just a misunderstanding of how the matrix stack works. You only ever have 1 active matrix. That is the matrix on top of the stack. So if m1 is on top of the stack and you call glTranslate, you still have m1 at the top of the stack but now it is multiplied by the matrix created by the glTranslate call. Those calls do not perform an implicit "glPushMatrix" as your question seemed to indicate. So to answer your question, m1 and m2 are not identical, m2 replaces m1, and yes it remains the top matrix on the stack because we did not call pushmatrix or popmatrix.
Also, the number of matrices in the stack has nothing to do with how many times your geometry will be drawn. Geometry is drawn when you call glVertex inside of a glBegin block, or when you call glDrawArrays or glDrawElements or related functions. The matrix at the top of the stack will then influence the position of the geometry in the "world" and thus on your screen, but it has no influence on the number of times the geometry is rendered.

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Quote:
Original post by jskywalker
Q1.

The matrix stack contains a set of matrix, only the top matrix (which is the current matrix) can be modified by glTranslatef , rotate or scale , is that correct? What about other old matrices ? They have been processed or waiting for being processed ?


Kinda. The matrices on the matrix stack are never modified. They're just pushed on or popped off. When you call glPushMatrix, you push a copy of the active matrix into the matrix stack. When you call glPopMatrix, you remove the top matrix from the stack and it becomes the active matrix. The matrix stack could remain empty during your entire render process and things would be fine.

The only old matrices that exist are the ones you've saved by pushing them to the matrix stack. Matrices on the stack are not processed at all unless you pop them and do some more rendering with them. Generally you would want to push a matrix if you plan to use it again later.

Quote:

Q2.

Similar question, see the diagram, after glLoadIdentity(), m2 becomes the current matrix, the top one in the matrix stack, am I correct ? if so, then using glTranslatef can affect the current matrix (m2), but not m1, correct ? Since m1 and m2 are identical, you will see (say the triangle) will be drawn twice.


No. glLoadIdentity() doesn't affect the matrix stack at all. Only glPushMatrix and glPopMatrix do. Also the triangle will only be drawn twice if you send the vertices to opengl twice.

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Quote:
Original post by CodeMunkie
Quote:
Original post by jskywalker
OK, let me use the following diagram to ask 2 more questions regarding the matrix thing.

A triangle matrix:
_ _
| 7 8 9 |
| -6 -4 -7 | original matrix (m1)
|_-9 -4 5 _|

*
_ _
| 1 0 1 |
| 0 1 0 | identity matrix ( glLoadIdentity(); )
|_ 0 0 1 _|

=
_ _
| 7 8 9 |
| -6 -4 -7 | current matrix (m2)
|_-9 -4 5 _|


Q1.

The matrix stack contains a set of matrix, only the top matrix (which is the current matrix) can be modified by glTranslatef , rotate or scale , is that correct? What about other old matrices ? They have been processed or waiting for being processed ?

Q2.

Similar question, see the diagram, after glLoadIdentity(), m2 becomes the current matrix, the top one in the matrix stack, am I correct ? if so, then using glTranslatef can affect the current matrix (m2), but not m1, correct ? Since m1 and m2 are identical, you will see (say the triangle) will be drawn twice.


A1.

Yes the matrix stack contains a set of matrices. Every time you call glPushMatrix(), a copy of the top matrix is pushed to the top of the stack. When you call glPopMatrix, the top matrix is removed from the stack and the matrix under it becomes the new top matrix. And yes, only the top matrix is modified by translate, rotate, scale AND glLoadIdentity. The other matrices are not at the top of the stack, so these calls have no effect on them.

A2.

The diagram is wrong. glLoadIdentity REPLACES the matrix at the top of the stack with the identity matrix (implied by the word "Load" in the function name). There is also a function called glLoadMatrix that allows you to replace the current matrix with an arbitrary one of your choosing. glRotate, Translate and Scale however will do multiplication. So m2 in your diagram would be the identity matrix. The rest of your question is just a misunderstanding of how the matrix stack works. You only ever have 1 active matrix. That is the matrix on top of the stack. So if m1 is on top of the stack and you call glTranslate, you still have m1 at the top of the stack but now it is multiplied by the matrix created by the glTranslate call. Those calls do not perform an implicit "glPushMatrix" as your question seemed to indicate. So to answer your question, m1 and m2 are not identical, m2 replaces m1, and yes it remains the top matrix on the stack because we did not call pushmatrix or popmatrix.
Also, the number of matrices in the stack has nothing to do with how many times your geometry will be drawn. Geometry is drawn when you call glVertex inside of a glBegin block, or when you call glDrawArrays or glDrawElements or related functions. The matrix at the top of the stack will then influence the position of the geometry in the "world" and thus on your screen, but it has no influence on the number of times the geometry is rendered.



Your answers are very clear, helpful. I now have no issues with understanding the concept of matrix stack and push / pop operations.

Only the identity matrix I am still not sure. I actually copied this diagram from my lecture note.

-----------------------------------------------
Identity Matrix
• Similarly to multiplication of real numbers, matrices have
inverses and identities.
• We know that any number times 1 will give us the original
number. Likewise, any square matrix times the identity matrix
of equal size equals the original matrix.
• We will see more of the identity matrix in future lectures.
-----------------------------------------------

Also from this link http://stackoverflow.com/questions/628796/what-does-glloadidentity-do-in-opengl. They explained the middle one

1 0 1
0 1 0
0 0 1

is the identity matrix. So I think maybe m2 (current matrix) is the result of loading identity matrix.

For matrix multiplication, glLoadIdentity replace current active / top matrix with identity matrix. But what is the purpose of using it in terms of drawing, if you say it resets the matrix to default state, then why we do that?

The identity matrix concept is very important for me to understand OpenGL. Please kindly note that I really need an accurate answer on this.

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I found an example of stack operations. I think I got the idea now this time, please confirm me / correct me if you can.

-------------------------------------------------------------------------------
The following illistrates the hierarchical stack method described in the above Robot example:
//...
glTranslatef(x,y,z); // 1) position for drawing torso
DrawTorso(); // 2) draw torso
glPushMatrix(); // 3) save torso matrix
glTranslatef(x,y,z); // 4) position for drawing arm
DrawArm(); // 5) draw arm
glPushMatrix(); // 6) save arm matrix
glTranslatef(x,y,z); // 7) position for drawing hand
DrawHand(); // 8) draw hand
glPushMatrix(); // 9) save hand matrix
glTranslatef(x,y,z); // 10) position for drawing first finger
DrawFinger(); // 11) draw first finger
glPopMatrix(); // 12) load hand matrix
glPushMatrix(); // 13) save hand matrix again
glTranslatef(x,y,z); // 14) position for drawing second finger
DrawFinger(); // 15) draw second finger
glPopMatrix(); // 16) load hand matrix
glPushMatrix(); // 17) save hand matrix again
glTranslatef(x,y,z); // 18) position for drawing third finger
DrawFinger(); // 19) draw third finger
glPopMatrix(); // 20) load hand matrix
glPopMatrix(); // 21) load arm matrix
glPopMatrix(); // 22) load torso matrix
------------------------------------------------------------------------

So for the matrix stack, you just add new matrix (operation) to the stack, openGL will handle the matrix multiplication for sure, but not in the stack.

Stack has a limit size. You push (add Matrix) and pop (load matrix) in the stack.

All matrices / operations in the stack eventually are processed for rendering, and stack remains empty during the rendering.

Still the issue with glLoadIdentity(), for above example, glLoadIdentity() applies the whole stack or just one Matrix / one operation ? If its latter, then which matrix should be the current matrix ?

Thanks again

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First off, I want to mention you are learning depricated OpenGL commands. OpenGL 3 has a different way of dealing with all this that may or may not be easier for you to understand.

You should be looking into learning some linear algebra, as it would make a lot of what is going on more clear.

But back to your way of doing it:
The opengl matrix stack is just a way to save your location in a hierarchy of transforms. It doesn't do anything, and isn't processed per-se.
On the model-view matrix, calling glLoadIdentity() causes drawing to move directly to the origin with no rotation. Pushing a matrix saves it, and poping it restores it. So if you load the identity (move to 0,0,0) then push, then translate left, then pop you are back to (0,0,0). If you didn't issue any draw commands, then you didn't really do anything.

Quote:

Still the issue with glLoadIdentity(), for above example, glLoadIdentity() applies the whole stack or just one Matrix / one operation ? If its latter, then which matrix should be the current matrix ?

glLoadIdentity applies to the top of the stack, and thus anything that happens after it. All the commands like glTranslate and glRotate just affect the top matrix in the stack. If you push, then loadidentity, you can pop and get back to whatever matrix you had before that.


As for the opengl 3 stuff, you only deal with sending a matrix to the shader. A matrix just encompases a transformation, scale, rotation, and skew. In opengl 3, you'd just make a matrix that represents the location and rotation you want, and send that to the GPU before sending the draw operation for the object you wanted to draw. The is more flexable (since you can make the shader do whatever you want with the data) and tends to be a bit faster (you can batch send matrices for a bunch of objects).

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Quote:

But back to your way of doing it:
The opengl matrix stack is just a way to save your location in a hierarchy of transforms. It doesn't do anything, and isn't processed per-se.
On the model-view matrix, calling glLoadIdentity() causes drawing to move directly to the origin with no rotation. Pushing a matrix saves it, and poping it restores it. So if you load the identity (move to 0,0,0) then push, then translate left, then pop you are back to (0,0,0). If you didn't issue any draw commands, then you didn't really do anything.


Example 1:

// drawing data
GLshort vertexArray1[8] = {0, 0, 5, 0, 0, 20, 5, 20};
glColor4f(1.0f, 1.0f, 0.0f, 1.0f);
glVertexPointer(2, GL_SHORT, 0, vertexArray1);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

// transformation
glLoadIdentity();
glRotatef(30.0f, 1.0f, 1.0f, 0);

Sorry I am not sure what u mean by "location", for above example, you have two set of locations. One is the original vertex location, one is the location after the transformation.

For above example, all openGL commands (drawing and transformation) go into the Matrix stack? Or just the transformation part? If so where OpenGL store the drawing part?

------------------------------------------------------------------------------

Example 2

glLoadIdentity();

// draw a rectangle
GLshort vertexArray[8] = {100, 100, 200, 100, 100, 300, 200, 300};
glVertexPointer(2, GL_SHORT, 0, vertexArray);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

glTranslatef(30.0f, 30.0f, 0);



This won't move the rectangle. For example 2, if all commands go into matrix stack, then they are added to the current matrix of the stack, aren't they? Since glTranslatef only work on current matrix, then why the rectangle won't move?



-----------------------------------------------------------------------------

Example 3

// rectangle 1
GLshort vertexArray1[8] = {100, 100, 200, 100, 100, 300, 200, 300};
glVertexPointer(2, GL_SHORT, 0, vertexArray1);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);


glTranslatef(30.0f, 30.0f, 0);

// rectangle 2
GLshort vertexArray2[8] = {100, 100, 200, 100, 100, 300, 200, 300};
glVertexPointer(2, GL_SHORT, 0, vertexArray2);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);


This time no glLoadIdentity(), but glTranslatef keep translate all rectangles. According to phresnel (OpenGL is a state machine. Every gl-call affects the following calls.), glTranslatef should only affect rectangle 2, but not rec 1.

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bear in mind that matrix stack operations have been depreciated a while ago and do not work in the opengl 3.0+ core profiles! better grab GLM or a similar library to do the job.

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Stop thinking in terms of a stack. Unless your code contains glPushMatrix and glPopMatrix, you won't even touch the matrix stack. Just think in terms of a single matrix transforming what you draw.
Quote:
Original post by jskywalker
Example 1:

// drawing data
GLshort vertexArray1[8] = {0, 0, 5, 0, 0, 20, 5, 20};
glColor4f(1.0f, 1.0f, 0.0f, 1.0f);
glVertexPointer(2, GL_SHORT, 0, vertexArray1);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

// transformation
glLoadIdentity();
glRotatef(30.0f, 1.0f, 1.0f, 0);

Sorry I am not sure what u mean by "location", for above example, you have two set of locations. One is the original vertex location, one is the location after the transformation.

For above example, all openGL commands (drawing and transformation) go into the Matrix stack? Or just the transformation part? If so where OpenGL store the drawing part?

First you draw your object, and then you set the matrix. Your translation will not affect the object, because you draw it before you set the matrix to contain the translation (possible exception; see point 3 below).
Quote:
Original post by jskywalker
Example 2

glLoadIdentity();

// draw a rectangle
GLshort vertexArray[8] = {100, 100, 200, 100, 100, 300, 200, 300};
glVertexPointer(2, GL_SHORT, 0, vertexArray);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

glTranslatef(30.0f, 30.0f, 0);



This won't move the rectangle. For example 2, if all commands go into matrix stack, then they are added to the current matrix of the stack, aren't they? Since glTranslatef only work on current matrix, then why the rectangle won't move?

You clear the matrix from doing any transformations since you load the identity matrix. Then you draw your object, and since the matrix is the identity matrix, there will be no transformation of the object. Then you set the translation, but that is too late, because you have already drawn the object.


Quote:
Original post by jskywalker
Example 3

// rectangle 1
GLshort vertexArray1[8] = {100, 100, 200, 100, 100, 300, 200, 300};
glVertexPointer(2, GL_SHORT, 0, vertexArray1);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);


glTranslatef(30.0f, 30.0f, 0);

// rectangle 2
GLshort vertexArray2[8] = {100, 100, 200, 100, 100, 300, 200, 300};
glVertexPointer(2, GL_SHORT, 0, vertexArray2);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);


This time no glLoadIdentity(), but glTranslatef keep translate all rectangles. According to phresnel (OpenGL is a state machine. Every gl-call affects the following calls.), glTranslatef should only affect rectangle 2, but not rec 1.

First frame, the matrix is set to the identity matrix (because that is the default value), so the first object will not be transformed in any way. Then you add a translation, and the second object is translated by 30.

Second frame, the matrix contains a translation of 30 from previous frame, so the first object is translated by 30. Then you add another 30, making the second object translate by 60.

Third frame, the first object is translated by 60 from previous frame, and the second object is translated by 90.

... ad infinitum. So you see, transformations only affect what is draw after the transformation is made. And the reason the first object is translated is because in the second frame, it is drawn after the second object (and, consequently, the translation) from the first frame. That is why they move, because you accumulate transformations. And that is also why the first object is 30 behind the second object, because you accumulate transformations between the two.

Now, the exception from the first point; since you don't reset the matrix before drawing the object, it is not possible to say what will happen to it. If this is the first frame, is is probably set to the identity matrix and the object will not be transformed. However, there may be some transformation set elsewhere in the program, for example by accumulating transformations from previous frame.

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Quote:
Original post by Vexator
bear in mind that matrix stack operations have been depreciated a while ago and do not work in the opengl 3.0+ core profiles! better grab GLM or a similar library to do the job.


I am actually learning OpenGL ES 1.1 for my windows mobile project. For desktop OpenGL I don't use. OpenGL 3.0 will affect OpenGL ES as well????

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Example 1:

// drawing data
GLshort vertexArray1[8] = {0, 0, 5, 0, 0, 20, 5, 20};
glColor4f(1.0f, 1.0f, 0.0f, 1.0f);
glVertexPointer(2, GL_SHORT, 0, vertexArray1);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

// transformation
glLoadIdentity();
glRotatef(30.0f, 1.0f, 1.0f, 0);

Quote:

First you draw your object, and then you set the matrix. Your translation will not affect the object, because you draw it before you set the matrix to contain the translation (possible exception; see point 3 below).


Hi Bob,

if I put above code in a loop, like your said, multi-frames, then the rectangle will rotate, am I correct?


Oh, I just tested it, it worked like that.


------------------------


[Edited by - jskywalker on August 28, 2010 10:58:40 AM]

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Thank you thank you everyone who helped and gave answers for my post.

And thank you Gamedev.net, great website for helping game developers.....

I now have clear understanding about matrix stack, identity and push pop concept.

One last question , I am sure :-) .

// drawing data
GLshort vertexArray1[8] = {0, 0, 5, 0, 0, 20, 5, 20};
glColor4f(1.0f, 1.0f, 0.0f, 1.0f);
glVertexPointer(2, GL_SHORT, 0, vertexArray1);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

// transformation
glRotatef(30.0f, 1.0f, 1.0f, 0);

For above example, all openGL commands (drawing and transformation) go into the current / active matrix of the matrix stack?

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This analogy isn't 100% accurate but think of the stack as a convenience to you, the programmer. Think of OpenGL calls (other than stack operations) as being stack-agnostic. When you push a matrix onto the stack, you're preserving it for later use. When you push a matrix off the stack, this becomes the 'active' matrix that is the recipient of all matrix operations (transate, loadidentity, etc.) and also the one that will affect proceeding drawing calls (even if they proceed in a following frame).

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Quote:
Original post by JackTheRapper
When you push a matrix onto the stack, you're preserving it (THE matrix you just pushed ???) for later use. When you push a matrix off the stack, this (THE matrix you just popped ???) becomes the 'active' matrix that is the recipient of all matrix operations (transate, loadidentity, etc.) and also the one that will affect proceeding drawing calls (even if they proceed in a following frame).


After reading your reply, I have clear my brain again about some basic concept.

According to openGL books and also here
http://gpwiki.org/index.php/OpenGL:Tutorials:Theory

I assume that the 'active' matrix you mentioned is the 'current' matrix.

---------------------------------------------------------------------------
OpenGL Matrix Stacks

The basic idea is that OpenGL maintains a stack of matrices whereas the top of the stack is the current matrix, and every time you use a transformation then it is applied to the current matrix.

glPushMatrix() copies the top of the stack, so directly after a push the top two entries of the stack are identical, equating to a ‘save and continue.’

glPopMatrix() would be the equivalent of ‘load’. This gets rid of the top matrix in the stack, and sets the matrix underneath it as the new current matrix. So, a glPopMatrix() could undo immediately the result of several or even hundreds of transformations.
---------------------------------------------------------------------------

We touch the matrix stack all the time, because the top of the matrix stack is the current matrix, and the current matrix stores current transformation. Any new transformation (matrix) goes into the current matrix ( multiply itself to current matrix), so the current matrix becomes newer.

Push Matrix, makes a copy of your current matrix, and places it on top of the stack, so any previous transformation is saved. In terms of drawing result, it saves what you have already drawn to the screen, and it makes sure that any new transformation will only affect new drawing.

Pop Matrix, gets rid of the top matrix (the one you just copied), nothing else.


glLoadIdentity() loads the Identity Matrix

1 0 1
0 1 0
0 0 1

(something like that)

and replaces the top matrix of the stack, so the identity matrix becomes the current matrix.
My last diagram is wrong, there is no multiplication involved, it simply replaces it.

Then Ricket (http://stackoverflow.com/questions/628796/what-does-glloadidentity-do-in-opengl) explained what happened after you load identity matrix.

--------------------------------------------------------------------------
But when one of these matrices are the identity matrix, the points are multiplied with the identity matrix and therefore are not changed, so the matrix has no effect; it does not translate the points, it does not rotate them, it leaves them as-is.
---------------------------------------------------------------------------

That's it. All my knowledge learnt so far for the last 2 days. Combining different view from different teacher from different site :-). I hope this time I get all correct.

If NOT, please correct me once more.

Have a good weekend everyone.

[Edited by - jskywalker on August 29, 2010 1:59:21 PM]

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Have a look at LIFO stacks (Last In First Out):

http://en.wikipedia.org/wiki/LIFO_(computing)

Lets say you have your frame rendering function something like this:

1) You call LoadIdentity()

No other matrices exist at this point. This call has set the current matrix to the identity matrix

2) You call Translate()

The current matrix (set to identity) is multiplied by the translation matrix created by the Translate() call. Any further geometry calls will be multiplied by this matrix. Still no other matrices exist on the stack

3) You call DrawSomething()

The vertices that make up the object in the DrawSomething() call are multiplied by the current matrix so will be translated by whatever values in the Translate() call

4) You call PushMatrix()

Now the current (translation) matrix is pushed onto the stack for later retrieval but is still the current matrix

5) You call DrawSomething() again

The vertices that make up the object in the DrawSomething() call are multiplied by the current matrix (still the translate matrix from step 2)

6) You call Rotate()

Now the current matrix (still the translation) is multiplied by a rotation matrix set by the Rotate() call. Any further geometry calls will be multiplied by this matrix

7) You call DrawSomething() again

The vertices that make up the object in the DrawSomething() call are multiplied by the current matrix (the translate * rotate matrix created in step 6)

8) You call LoadIdentity()

You have now set the current matrix the identity matrix, i.e. it is no longer the translate * rotate matrix. Any further geometry calls will be multiplied by this matrix. However, unlike step 1, we still have the original translate matrix set in step 2 on the stack, waiting to be used again if we so desire)

9) You call DrawSomething() again

The vertices that make up the object in the DrawSomething() call are multiplied by the current matrix (identity)

10) You call PopMatrix()

Now the current matrix is set to the matrix we popped off the stack. There's only one matrix on the stack, the translate matrix set in step 2. Any further geometry calls will be multiplied by this matrix. Now that it's been popped off there are now no matrices on the stack


11) You call DrawSomething() again

The vertices that make up the object in the DrawSomething() call are multiplied by the current matrix so will be translated by whatever values in the Translate() call from step 2

And so on. You can nest push/pop calls to build up a hierarchy of transformations. The matrix operation calls will only affect the current matrix. The stack may or may not contain any matrices. It can be used to preserve the current matrix state so you can do further matrix operations and revert back to whatever matrix states you've put on the stack.

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      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
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      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains two samples, asteroids performance benchmark and example Unity project that uses Diligent Engine in native plugin.
      AntTweakBar sample is Diligent Engine’s “Hello World” example.

       
      Atmospheric scattering sample is a more advanced example. It demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

      Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

      Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

      Future Work
      The engine is under active development. It currently supports Windows desktop, Universal Windows and Android platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and support for more platforms is planned.
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      I'd really love to hear anyone's thoughts on this, any suggestions on what features you'd want to see in a library like this? 
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