2D textures with OpenGL and GLUT

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2D textures with OpenGL and GLUT

Elise Huard
Hi,
I've been trying to get a texture mapped onto a circle.
I've pounded my head against this problem for a good while,
been reading lots of posts about how to do it in C,
reproduced those as much as possible,
but the best I get is a fairly distorted texture
 (I'm working with a basic checkerboard as displayed in the Redbook4
examples of GLUT,
just to make sure I've got it right before moving on to actual images).
Code here:
loading texture
https://github.com/elisehuard/game-prototype/blob/master/src/Main.hs#L90
using the texture in a circle (mapping):
https://github.com/elisehuard/game-prototype/blob/master/src/Main.hs#L130

the result is here
https://github.com/elisehuard/game-prototype/blob/master/images/texture.jpg

(as you can see, it is not a checkerboard)
My aim is to make a 2D game to start with, so the perspective is orthogonal,
I don't know if that helps.

This may be due to my being a noob with things OpenGL -
I hope anyone can help me solve this, textures are a pretty important part.

Thank you,

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Re: 2D textures with OpenGL and GLUT

Jason Dagit-3



On Wed, Jul 2, 2014 at 8:06 AM, Elise Huard <[hidden email]> wrote:
Hi,
I've been trying to get a texture mapped onto a circle.
I've pounded my head against this problem for a good while,
been reading lots of posts about how to do it in C,

For folks who are experienced with low-level Haskell I actually recommend they use the OpenGLRaw library as it matches C examples more closely. Although, if you're a Haskell beginner that may just be an extra hassle.
 
reproduced those as much as possible,
but the best I get is a fairly distorted texture
 (I'm working with a basic checkerboard as displayed in the Redbook4
examples of GLUT,
just to make sure I've got it right before moving on to actual images).
Code here:
loading texture
https://github.com/elisehuard/game-prototype/blob/master/src/Main.hs#L90
using the texture in a circle (mapping):
https://github.com/elisehuard/game-prototype/blob/master/src/Main.hs#L130

It's hard to say what's going on with all those unsafeCoerce in there.



the result is here
https://github.com/elisehuard/game-prototype/blob/master/images/texture.jpg

(as you can see, it is not a checkerboard)
My aim is to make a 2D game to start with, so the perspective is orthogonal,
I don't know if that helps.

Lots of things can be orthogonal, but I believe in this case that orthographic is the word you want :)



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Re: 2D textures with OpenGL and GLUT

Elise Huard
Hi Jason,
thanks for taking the time to answer.
So trying to read between the lines:
- do you have any _working_ examples of texture mapping with OpenGLRaw
and low-level Haskell I could have a look at? (anything would help at
this point)
- do you think the unsafeCoerce (which I borrowed from examples) that
transform CpFloat to GLdouble might be the issue, or is this just a
general stylistic comment?
Thank you,

Elise

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Re: 2D textures with OpenGL and GLUT

Jason Dagit-3
In reply to this post by Jason Dagit-3



On Wed, Jul 2, 2014 at 9:33 AM, Elise Huard <[hidden email]> wrote:
Hi Jason,
thanks for taking the time to answer.
So trying to read between the lines:
- do you have any _working_ examples of texture mapping with OpenGLRaw
and low-level Haskell I could have a look at? (anything would help at
this point)

I do. I converted some of the old nehe tutorials: https://github.com/dagit/nehe-tuts

If you look through the history you can find examples that use a very very old version of OpenGL (instead of OpenGLRaw). The OpenGL style in those examples is old and no longer the recommended way. The examples use immediate mode which is now deprecated in favor of shaders.

I think lesson 6 makes a cube and throws some textures on it. The lesson numbers are meant to match these:

 
- do you think the unsafeCoerce (which I borrowed from examples) that
transform CpFloat to GLdouble might be the issue, or is this just a
general stylistic comment?

Not just stylistic. As for correctness, I think the answer varies by platform. Specifically, I think CFloat is not required to match GLdouble in size (in fact, doubles should be expected to be twice as big). While both types should correspond to IEEE 754 floats the bit width is very likely different and that could easily cause the interpretation of the values to go wonky. Imagine passing 64bits when only the first 32bits correspond to a valid number or truncating a 64bit value to 32bits.

Which examples use unsafeCoerce for this? I'm just sort of curious so I can recommend people not to use them :)


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Re: 2D textures with OpenGL and GLUT

Elise Huard-2
I wanted to thank you, because pointing to unsafeCoerce was a nice bit
of serendipity.  I'll admit I thought it was nitpicking initially, but
I did a quick pass at cleaning things up last week after your mail
(not perfect, but this was a prototype so I'm not looking for pretty):

https://github.com/elisehuard/game-prototype/commit/5cf85fa60f325dbe610b5e1f017c9427e304daf6

and that fixed the texture mapping!
Consider me informed on that issue :)

(as for which example, I used Redbook4 but I admit to extrapolating a
little, this was my mistake)

Thank you,

On 2 July 2014 13:11, Jason Dagit <[hidden email]> wrote:

>
>
>
> On Wed, Jul 2, 2014 at 9:33 AM, Elise Huard <[hidden email]> wrote:
>>
>> Hi Jason,
>> thanks for taking the time to answer.
>> So trying to read between the lines:
>> - do you have any _working_ examples of texture mapping with OpenGLRaw
>> and low-level Haskell I could have a look at? (anything would help at
>> this point)
>
>
> I do. I converted some of the old nehe tutorials:
> https://github.com/dagit/nehe-tuts
>
> If you look through the history you can find examples that use a very very
> old version of OpenGL (instead of OpenGLRaw). The OpenGL style in those
> examples is old and no longer the recommended way. The examples use
> immediate mode which is now deprecated in favor of shaders.
>
> I think lesson 6 makes a cube and throws some textures on it. The lesson
> numbers are meant to match these:
>   * http://nehe.gamedev.net/tutorial/lessons_01__05/22004/
>   * http://nehe.gamedev.net/tutorial/lessons_06__10/17010/
>
>
>>
>> - do you think the unsafeCoerce (which I borrowed from examples) that
>> transform CpFloat to GLdouble might be the issue, or is this just a
>> general stylistic comment?
>
>
> Not just stylistic. As for correctness, I think the answer varies by
> platform. Specifically, I think CFloat is not required to match GLdouble in
> size (in fact, doubles should be expected to be twice as big). While both
> types should correspond to IEEE 754 floats the bit width is very likely
> different and that could easily cause the interpretation of the values to go
> wonky. Imagine passing 64bits when only the first 32bits correspond to a
> valid number or truncating a 64bit value to 32bits.
>
> Which examples use unsafeCoerce for this? I'm just sort of curious so I can
> recommend people not to use them :)
>
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Re: 2D textures with OpenGL and GLUT

Jason Dagit-3



On Tue, Jul 8, 2014 at 9:20 AM, Elise Huard <[hidden email]> wrote:
I wanted to thank you, because pointing to unsafeCoerce was a nice bit
of serendipity.  I'll admit I thought it was nitpicking initially, but
I did a quick pass at cleaning things up last week after your mail
(not perfect, but this was a prototype so I'm not looking for pretty):

https://github.com/elisehuard/game-prototype/commit/5cf85fa60f325dbe610b5e1f017c9427e304daf6

and that fixed the texture mapping!
Consider me informed on that issue :)

Cool! By the way, the 'proper way' to convert between Double and CDouble is to use realToFrac:

Prelude Foreign.C.Types λ> realToFrac (1 :: Double) :: CDouble
1.0

In specific case like this (Double <-> CDouble) it should be equivalent (but as a user of the library you shouldn't depend on know that). In some cases it can be a performance hit, but that's what RULEs pragmas are for. Moreover, some versions of the Haskell OpenGL bindings made it so that GLdouble was not the same type as CDouble. So the safest way is to use realToFrac and depend on the library writer to provide RULEs that make it efficient.

I hope that helps,
Jason



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Re: 2D textures with OpenGL and GLUT

Elise Huard-2
Definitely, I want to avoid unsafeCoerce completely.
thanks,

Elise

On 8 July 2014 12:32, Jason Dagit <[hidden email]> wrote:

>
>
>
> On Tue, Jul 8, 2014 at 9:20 AM, Elise Huard <[hidden email]> wrote:
>>
>> I wanted to thank you, because pointing to unsafeCoerce was a nice bit
>> of serendipity.  I'll admit I thought it was nitpicking initially, but
>> I did a quick pass at cleaning things up last week after your mail
>> (not perfect, but this was a prototype so I'm not looking for pretty):
>>
>>
>> https://github.com/elisehuard/game-prototype/commit/5cf85fa60f325dbe610b5e1f017c9427e304daf6
>>
>> and that fixed the texture mapping!
>> Consider me informed on that issue :)
>
>
> Cool! By the way, the 'proper way' to convert between Double and CDouble is
> to use realToFrac:
>
> Prelude Foreign.C.Types λ> realToFrac (1 :: Double) :: CDouble
> 1.0
>
> In specific case like this (Double <-> CDouble) it should be equivalent (but
> as a user of the library you shouldn't depend on know that). In some cases
> it can be a performance hit, but that's what RULEs pragmas are for.
> Moreover, some versions of the Haskell OpenGL bindings made it so that
> GLdouble was not the same type as CDouble. So the safest way is to use
> realToFrac and depend on the library writer to provide RULEs that make it
> efficient.
>
> I hope that helps,
> Jason
>
>
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