Help with texture mapping [newbie]
Author
Hi All:
I am having some trouble with displaying a simple bitmap as a texture on a square 2D polygon displayed on the top left corner of a window.
I have:
glOrtho(-1.0, 1.0, 1.0, -1.0, 1.0, -1.0);
so left: -1.0, right 1.0, top -1.0, bottom 1.0.
I draw the square polygon without any troubles as:
glBegin(GL_POLYGON);
glVertex2d(-1.0, -1.0);
glVertex2d(-0.5, -1.0);
glVertex2d(-0.5, -0.5);
glVertex2d(-1.0, -0.5);
glEnd();
When I enter the texture co-ordinates however, I do not see the texture drawn as expected (I am expecting to just see the whole texture covering the whole polygon. I followed Gamedev's suggestion at http://www.gamedev.net/reference/articles/article947.asp re texture co-ordinates)
Here is the code i use to draw the texture-mapped polygon:
glBegin(GL_POLYGON);
glTexCoord2d(0.0, 1.0);
glVertex2d(-1.0, -1.0);
glTexCoord2d(1.0, 1.0);
glVertex2d(-0.5, -1.0);
glTexCoord2f(1.0, 0.0);
glVertex2d(-0.5, -0.5);
glTexCoord2f(0.0, 0.0);
glVertex2d(-1.0, -0.5);
glEnd();
Any one who can help me get these co-ordinates straight because I am getting a bit confused?!
Thanks
Duncan Camilleri
What does the texture look like, though? Does it render at all?
I'm a little confused by using a negative value for the top in glOrtho. Although I assume there is a reason you have done this, I have never personally seen it (usually I see people put 0,0 in the upper left, and screenwidth/screenheight in the lower right). I know negative values are allowed for the z-clipping planes, however I'm not 100% certain on the others. I guess it shouldnt be a problem, since its just a matrix multiplication, but it may be worth a try.
Perhaps try glOrtho(0.0, 2.0, 2.0, 0.0, -1.0, 1.0) and shift your polygon over, simply to eliminate this as a possible problem.
Additionally, I believe polygons are usually wound counter-clockwise for culling reasons in OpenGL. So you may want to change the order you draw it in.
I am not certain what you're problem is, so these are just a few things you may wish to try while waiting for more replies.
Good luck.
I'm a little confused by using a negative value for the top in glOrtho. Although I assume there is a reason you have done this, I have never personally seen it (usually I see people put 0,0 in the upper left, and screenwidth/screenheight in the lower right). I know negative values are allowed for the z-clipping planes, however I'm not 100% certain on the others. I guess it shouldnt be a problem, since its just a matrix multiplication, but it may be worth a try.
Perhaps try glOrtho(0.0, 2.0, 2.0, 0.0, -1.0, 1.0) and shift your polygon over, simply to eliminate this as a possible problem.
Additionally, I believe polygons are usually wound counter-clockwise for culling reasons in OpenGL. So you may want to change the order you draw it in.
I am not certain what you're problem is, so these are just a few things you may wish to try while waiting for more replies.
Good luck.
Having a negative value in glOrtho() as such simply means that the coordinates start from the top-left, rather than the bottom-left, I believe.
Also, change your texture polygon code to this:
glBegin gl_Quad
glTexCoord2f 0,0
glVertex2f 0,0
glTexCoord2f 1,0
glVertex2f 1,0
glTexCoord2f 1,1
glVertex2f 1,1
glTexCoord2f 0,1
glVertex2f 0,1
glEnd
Does that work? Are you binding the texture before hand? Have you verified that the texture has loaded correctly?
Also, change your texture polygon code to this:
glBegin gl_Quad
glTexCoord2f 0,0
glVertex2f 0,0
glTexCoord2f 1,0
glVertex2f 1,0
glTexCoord2f 1,1
glVertex2f 1,1
glTexCoord2f 0,1
glVertex2f 0,1
glEnd
Does that work? Are you binding the texture before hand? Have you verified that the texture has loaded correctly?
Don't forget to enable textures!: glEnable(GL_TEXTURE_2D)
I've forgotten to do that many times to great and embarrassing frustration.
I've forgotten to do that many times to great and embarrassing frustration.
Author
All your comments are much appreciated.
I tried to apply the same thing counter clockwise with 0.0 replacing -1.0 but all is in vain.
The image does get rendered, but is being drawn incorrectly (unexpected rotation and skewing is being done) so I do not think the problem is with binding or enabling textures.
Something very funny that I note, is that I am trying this with two bitmap versions. A chequered bitmap generated via code (as seen in some external sample) which works perfectly. However, when I simply replace the bit data with that of a bitmap loaded from file, then the problems start to appear. For both bitmaps, the data stored in memory is formatted as 32 bits per pixel.
Thanks
Duncan
I tried to apply the same thing counter clockwise with 0.0 replacing -1.0 but all is in vain.
The image does get rendered, but is being drawn incorrectly (unexpected rotation and skewing is being done) so I do not think the problem is with binding or enabling textures.
Something very funny that I note, is that I am trying this with two bitmap versions. A chequered bitmap generated via code (as seen in some external sample) which works perfectly. However, when I simply replace the bit data with that of a bitmap loaded from file, then the problems start to appear. For both bitmaps, the data stored in memory is formatted as 32 bits per pixel.
Thanks
Duncan
Author
Aaaaahh ok I got the solution!
From the text, I have read how the width and height should be 2n + 2 so I created the bitmap to be 66X66 (for the +2). Changing the size of my texture bitmap back to 64X64 solved the problem.
I also am confirming that with -1.0 in glOrtho and when drawing the polygon clockwise rather than counter clockwise, still works fine!
I thank you all for your input!
Regards
duncan
From the text, I have read how the width and height should be 2n + 2 so I created the bitmap to be 66X66 (for the +2). Changing the size of my texture bitmap back to 64X64 solved the problem.
I also am confirming that with -1.0 in glOrtho and when drawing the polygon clockwise rather than counter clockwise, still works fine!
I thank you all for your input!
Regards
duncan
This topic is closed to new replies.
Advertisement
Popular Topics
Advertisement
