Pass multiple texture coordinates to my shader

OpenGL
1

Hello,

I have a question that I cannot figure out on my own.
Maybe somebody could give me a hint or a link.
Any help would be appreciated!
Thanks in advance.

What I want to do:

I want to draw a little 2D sprite on the screen.
When drawing the sprite I want to take the color information from one texture (“tex”) and the alpha information from another one (“mask”).

The fragment shader right now looks like this:

uniform sampler2D tex; // providing RGB
uniform sampler2D mask; // providing alpha

void main()
{
	vec4 texel = texture2D(tex, gl_TexCoord[0].st);
	vec4 mask = texture2D(mask, gl_TexCoord[0].st);

	texel.a = mask.a;

	gl_FragColor = texel;
}

Here is my problem:

The texture coordinates in texture “tex” and the ones in “mask” are not the same.
Unfortunately I don’t know how to pass two different texture coordinates to my shader.
Right now I pass texture coordinates like this:

gl2.glBegin(GL2.GL_QUADS);
{
	gl2.glTexCoord2f(tx1, ty1);
	gl2.glVertex3f(qx1, qy1, 0.0f);

	gl2.glTexCoord2f(tx2, ty2);
	gl2.glVertex3f(qx2, qy1, 0.0f);

	gl2.glTexCoord2f(tx2, ty2);
	gl2.glVertex3f(qx2, qy2, 0.0f);

	gl2.glTexCoord2f(tx1, ty2);
	gl2.glVertex3f(qx, qy2, 0.0f);
}
gl2.glEnd();

So I guess I will need to enhance this code somehow but I cannot find out how.
I’m using JOGL (java bindings for OpenGL) if that matters.

Thanks a lot for your time and help!
Greetings Xoric

2

Hi,

try using
glMultiTexCoord2f( GL_TEXTURE0, s1, t1);
glMultiTexCoord2f( GL_TEXTURE1, s2, t2);

and access them in the sahder as gl_TexCoord[0].st and gl_TexCoord[1].st .

Alternatively (based on the target hardware) you could use generic vertex attributes & VBOs instead of using the old immediate mode and the fixed number of texture coordinates.
(The immediate mode is still fine for simple applications where performance is not a concern or if the app has to run on hardware older than 5 years but it’s probably not the way to start learning OpenGL in 2012…).

3

Technically that shader looks correct, though I’d not the use the name “mask” both for the uniform and the vec4.

I once had a similar problem in Mesa GLSL, there changing to


texel = vec4(texel.rgb, mask.a);

fixed it.

4

If the alpha mask matches the sprite then just sample the mask texture with the same coordinates.

Otherwise use glmultitexcoord or an additional shader attribute (the new preferred way) to send a new set of coordinates.

5

Hello!

Your code was exactly what I was looking for menzel.
I could not get it to work the way I intended yet but I will figure it out soon. Thanks a lot!

Also thanks for your hint mbentrup.
I edited the shader. Thanks!

The alpha mask does not have the same coordinates dorbie.
That’s why I will use glMultiTexCoord. Thank you!

Greetings
Xoric

6

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