Problem with 1dTexture as lookup table

OpenGL
1

Hello there,

i would like to draw the grayscale color 255 as red and i would like to do this with a FragementShader, but it dosen’t work.

I define a lookup texture like this:

	glGenTextures(1, &m_tex[0]);
	glBindTexture(GL_TEXTURE_1D, m_tex[0]);
	// setup texture
	glTexParameteri (GL_TEXTURE_1D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
	glTexParameteri (GL_TEXTURE_1D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);


	float colors[256][3];
	for (int i = 0; i < 256; i++)
	{
	if (i == 255)
		{
			colors[i][0] = i / 255.;
			colors[i][1] = 0.;
			colors[i][2] = 0.;
		}
		else
		{

			colors[i][0] = i / 255.;
			colors[i][1] = i / 255.;
			colors[i][2] = i / 255.;

		}
	}

	glTexImage1D(GL_TEXTURE_1D, 0, GL_RGB, 256, 0 , GL_RGB, GL_FLOAT, colors);

After this i create my 2D image grayscale texture:

		// texture erstellen
		glGenTextures(1, &m_tex[1]);
		glBindTexture(GL_TEXTURE_2D, m_tex[1]);

		// setup texture
		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

		// copy image pixel to texture
		glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, m_gray.width(), m_gray.height(), 0,
				GL_LUMINANCE, GL_UNSIGNED_BYTE, m_gray.bits());

Then i define the shaders Fragment and Vertex:

const char *vsrc =
	         "void main(void)
"
	         "{
"
	         "    gl_Position = ftransform();
"
	         "gl_TexCoord[0] = gl_MultiTexCoord0;
"
	         "}
";


const char *fsrc =
		"uniform sampler2D sample;
"
		"uniform sampler1D lookup;
"
		"void main(void)
"
		"{
"
		"	gl_FragColor = texture1D(lookup, texture2D(sample,gl_TexCoord[0].st).r);
"
		"}
";

Now i use the Qt QGLWidget to draw the texture. I compile now the
code and bind it to the card.

m_program = new QGLShaderProgram(this);
	m_program->addShader(vshader);
	m_program->addShader(fshader);
	m_program->link();
	m_program->setUniformValue("lookup", m_tex[0]);
	m_program->setUniformValue("sample", m_tex[1]);
	m_program->bind();

And this is my draw function:

paintGL()
{

	glEnable(GL_TEXTURE_2D);
	glBegin(GL_QUADS);
		// clockwise
		glTexCoord2f(0.0f, 0.0f); glVertex2f(-1.0f, 1.0f); 	// oben links
		glTexCoord2f(1.0f, 0.0f); glVertex2f(1.0f, 1.0f); 	// oben rechts
		glTexCoord2f(1.0f, 1.0f); glVertex2f(1.0f, -1.0f);	// unten rechts
		glTexCoord2f(0.0f, 1.0f); glVertex2f(-1.0f, -1.0f);	// unten links
	glEnd();
	glDisable(GL_TEXTURE_2D);

}

Can anyone help me what did i wrong? I see only the 1D lookup texture in my Widget and the color which should be red is black.

Best regards,

Treehouse

2

How about a picture? It’s not obvious from your description what you are seeing. Just that you “aren’t” seeing the all white color show up red.

Could be you’re using CLAMP rather than CLAMP_TO_EDGE, so you’re getting the texture border mixed in. To fix:


glTexParameteri( GL_TEXTURE_2D , GL_TEXTURE_WRAP_S    , GL_CLAMP_TO_EDGE ) ;
glTexParameteri( GL_TEXTURE_2D , GL_TEXTURE_WRAP_T    , GL_CLAMP_TO_EDGE ) ;

And of course, for a 1D texture, omit the 2nd line and change 2D to 1D.

3

Hello Dark Photon,

if i don’t bind the shader i see my grayscale image, normal with no problems.

When i change the draw function in this way also with unbind shader:

glEnable(GL_TEXTURE_1D);
	glBegin(GL_QUADS);
		// clockwise
		glTexCoord2f(0.0f, 0.0f); glVertex2f(-1.0f, 1.0f); 	// oben links
		glTexCoord2f(1.0f, 0.0f); glVertex2f(1.0f, 1.0f); 	// oben rechts
		glTexCoord2f(1.0f, 1.0f); glVertex2f(1.0f, -1.0f);	// unten rechts
		glTexCoord2f(0.0f, 1.0f); glVertex2f(-1.0f, -1.0f);	// unten links
	glEnd();
	glDisable(GL_TEXTURE_1D);

Then i see my 1D Texture with the grayscales from black to white and the last line of pixels are red.

And when bind the shader and i use my 2D draw function. I don’t see my grayscale image instead i see the 1D texture from black to white but the last line that must be red is black.

Best regards,

Treehouse

4

You need to pass the active texture where the texture is bound and not the tex id that your texture stores when u pass the uniform to the shader.
Change this line


m_program->setUniformValue("lookup", m_tex[0]);
m_program->setUniformValue("sample", m_tex[1]);

to this


m_program->setUniformValue("lookup", 0);
m_program->setUniformValue("sample", 1);

And then when passing the texture make sure that the texture is bound to the appropriate active texture target.


glGenTextures(1, &m_tex[0]);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_1D, m_tex[0]);
// setup texture
glTexParameteri (GL_TEXTURE_1D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri (GL_TEXTURE_1D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
...

and the same steps for the other texture


// texture erstellen
glGenTextures(1, &m_tex[1]);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, m_tex[1]);

// setup texture
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

...

See if this helps.

5

Hello mobeen,

you made my day. It works thank you very much for this advice.

Best regards,

Treehouse

6

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