2D perspective correct texturing ?

[SesillaAndromeda]

The first one! The texture projected like pseudo 3d!

Sesilla

[phibermon]

If you're in OpenGL then you're providing texture coordinates for the quad. Perspective correction is automatic. You provide coordinates between 0.0 and 1.0 for each of your vertices. To produce an image like you've posted you have to scale the texture coordinates in relation to your vertex coordinates, or in much older versions of GL you can use automatic texture coordinate generation which in 2D will produce the same result.

The solution for 'perspective' in 2D as you desire is to simply leave your texture coordinates at each corner of the texture while altering the vertices of the quad.

Input the vertex coordinates that would give you a quad like in the image you have provided and in counter-clockwise order (OpenGL defaults to counter-clockwise winding to determine front facing polygons) set your texture coordinates as (x:0,y:0) (x:0,y:1) (x:1,y:1) (x:1,x:0)

As long as you keep your texture coordinates the same, you can draw your (convex) quad at any combination of vertices and openGL will 'skew' the texture automatically.

[SesillaAndromeda]

Hi phibermon,

with this code:

glBegin(GL_QUADS);
        
glTexCoord2f(0, 0);

glVertex2f  (500, 200);

glTexCoord2f(0, 1);
glVertex2f  (600,  200);

glTexCoord2f(1, 1);
glVertex2f  (1000,  700);

glTexCoord2f(1, 0);
glVertex2f  (50, 700);

this is the rendering:



Browsing the web i found the Opengl glTexCoord4f function, but it is not clear about use it.

Thanks
Sesilla

[phibermon]

Your vertices are in clockwise order and your texture coordinates are in counter-clockwise order, the GPU actually draws two triangles and not a quad and because you have opposite winding, the two triangles are getting different relative texture coordinates. Always draw two triangles instead of one quad so this fact is always present in your mind and such issues become more obvious. It's better for all kinds of technical reasons that you don't need to learn if you don't want to. try the following instead :

glBegin(GL_TRIANGLES);
        
glTexCoord2f(0, 0);
glVertex2f(500, 200);

glTexCoord2f(0, 1);
glVertex2f(50, 700);

glTexCoord2f(1, 1);
glVertex2f  (1000,  700);


glTexCoord2f(1, 1);
glVertex2f  (1000,  700);

glTexCoord2f(1, 0);
glVertex2f  (600,  200);


glTexCoord2f(0, 0);
glVertex2f(500, 200);

glEnd();

Counter-clockwise always for coordinates. OpenGL and other API's like Direct3D use what is called the 'winding order' to determine if a one sided polygon is facing you (the 'camera') ( one sided polygon = default OpenGL state - simply meaning that it will be invisible if viewed from the other side. This implies that you can render two-sided polygons that are visible from both sides and this is true. This would seem preferable as you don't have to care about winding but in reality it slows down rendering as discarding polygons that can't be seen is one of the main principals behind rendering things more quickly. In 2D this isn't really an issue as you'll never be 'looking' from the other side of the polygon but it's best practice and should you move to 3D you'll already be doing things correctly)

[SesillaAndromeda]

Hi

thanks for your reply, but with your code this is the result:



Have i done something wrong?

Thanks
Sesilla

[phibermon]

Ah my apolgies, it's been a long time since I worked in immediate GL mode. The way this would be done now is by using a projection sampler in a shader. However, I believe that immediate GL had a nice friendly trick that does some magic for you. Add the following somewhere in your initialization code, before you call GL_Begin() :

glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);

Hopefully my memory has not failed me and this should produce your desired result.

[phibermon]

Bare in mind that GLHINT may not produce this same magic on all cards. The correct way is to use 4D texture coordinates so you can control this behaviour in the correct manner.

See this page :

https://home.xyzw.us/~cass/qcoord/

My guess is that some drivers will handle this and other will not so by controlling all four parameters of the texture coords you can guarantee desired behaviour on all cards.

[SilverWarior]

Didn't you also messed up with the coordinates here? Specifically for second triangle?

glBegin(GL_TRIANGLES);

//FirstTriangle

//TopLeft        
glTexCoord2f(0, 0);
glVertex2f(500, 200);

//BottomLeft
glTexCoord2f(0, 1);
glVertex2f(50, 700);

//BottomRight
glTexCoord2f(1, 1);
glVertex2f  (1000,  700);


//SecondTriangle

//BottomRight
glTexCoord2f(1, 1);
glVertex2f  (1000,  700);

//TopRight
glTexCoord2f(1, 0);
glVertex2f  (600,  200);

//TopLeft
 glTexCoord2f(0, 0);
glVertex2f(500, 200);

glEnd();

Shouldn't the order of coordinates for second triagnle be TopLeft, BottomRight, TopRight?

[Ñuño Martínez]

Why do I have feeling that Nuno spends too much time on communities like Stack Overflow?

I don't know, because I never used Stack Overflow, and I don't know if any other or the communities I spend too much time on are like that as I never used it.

I would expect such answer on the Stack Overflow where most people blindly tends to answer the question that was asked and not the question that was intended to be asked.

I can answer this:

That's because, in my experience, the question that was intended to be asked is often not the actual question that should be asked. And if I've understood correctly the other messages, this is one of these cases. (Please, correct me if I'm wrong in this: I'm still learning English)

So Nuno can you please provide short example for rendering skew quad using OpenGL?

Of course I can, but it would be too similar to the one provided by phibermon.

[Disclaimer]: I must apoligize. Some times I can be too much sarcastic. Please, no offense intended. If you think I've exceed the limit, please tell me and I'll try to keep my responses under control. Really.

Didn't you also messed up with the coordinates here? Specifically for second triangle?

...

Shouldn't the order of coordinates for second triagnle be TopLeft, BottomRight, TopRight?

Yes, on the first triangle is counter-clockwise and the second is clockwise. Both them should have the same "wise" to use back face culling correctly.

[SesillaAndromeda]

Hi Ñuño,

sorry for may question, but I need only help for my problem. That's it!

Regards
Sesilla