Create a new texture from two blended textures

Graphics and GPU Programming Programming

Started by grasshop September 08, 2012 01:57 AM

1 comment, last by grasshop 11 years, 7 months ago

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September 08, 2012 01:57 AM

Hi guys, I am trying to create a new texture from a background texture but with it's alpha values set from the colour gradient of another texture. Ie like this:

This is a framework of the C++ code I am using:



unsigned tex_aft(int tex, int copy_tex)

{

	//tex is the texture to set the alpha to, copy_tex is the texture to take the colour gradients from for the alpha values

	GLuint texture = tex, copy_texture = copy_tex;

	glPushAttrib(GL_CURRENT_BIT | GL_COLOR_BUFFER_BIT | GL_ENABLE_BIT);

	glColor4f(1,1,1,1);

	glClear(GL_COLOR_BUFFER_BIT);

	glDisable(GL_BLEND);

	glBindTexture(GL_TEXTURE_2D, texture);

	int w, h;

	glGetTexLevelParameteriv(GL_TEXTURE_2D,0,GL_TEXTURE_WIDTH, &w);

	glGetTexLevelParameteriv(GL_TEXTURE_2D,0,GL_TEXTURE_HEIGHT, &h);

	glEnable(GL_BLEND);

	glBlendFunc(GL_ONE_MINUS_DST_ALPHA, GL_SRC_COLOR);

	glBindTexture(GL_TEXTURE_2D, copy_texture);

	char* bitmap = new char[(h<<(lgpp2(w)+2))|2];  //lgpp2() function is used to get powers of two

	glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA, GL_UNSIGNED_BYTE, bitmap);

	GLuint tex_new;

	glGenTextures(1, &tex_new);

	glBindTexture(GL_TEXTURE_2D, tex_new);

	glTexImage2D(GL_TEXTURE_2D, 0, 4, w, h, 0, GL_RGBA, GL_UNSIGNED_BYTE, void(bitmap));

	delete[] bitmap;

	glPopAttrib();

	return tex_new;

}

But from what I know you can't use glBlendFunc to blend bound textures like this, all this code winds up doing is returning a straight replica of the copy_tex. So does anybody here please know what code I could use to blend the two textures like I have described and create a new texture from this?

Thanks for any help.

larspensjo

1,561

September 08, 2012 08:10 AM

It depends a little on your requirements. If you only need to do it once, you may as well do it "off-line". That is, you can do it yourself without using OpenGL.

If you have real-time requirements, using OpenGL to do it seems like a good idea. The usual solution is to use a Frame Buffer Object.

However, I think you should to abandon the deprecated legacy opengl. With shaders, you get it exactly the way you want it.