How do I read the color of a pixel in opengl

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I am trying to read the color of a pixel. but for some reason it doesn't work

HERE'S THE CODE :

C++

ModelClassObject.UseShader(SelectionRender.GetShaderProgram());
			glUniformMatrix4fv(SelectionRender.GetUnifromModelID(), 1, GL_FALSE, glm::value_ptr(Model));
			glUniformMatrix4fv(SelectionRender.GetUnifromProjectionID(), 1, GL_FALSE, glm::value_ptr(Projection));
			glUniformMatrix4fv(SelectionRender.GetUniformView(), 1, GL_FALSE, glm::value_ptr(Camera.CalculateViewMatrix()));
			ModelClassObject.DrawTriangles();
			glFlush();
			glFinish();
			glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
			unsigned char data[3];
			glReadPixels(0, winHeight, winWidth, winHeight, GL_RGB, GL_UNSIGNED_BYTE, data);
			std::cout 
				<< "  R  "
				<< data[0]
				<< "  G  "
				<< data[1]
				<< "  B  "
				<< data[2] 
				<< std::endl;

OUTPUT :
data[0] = 0xcc
data[1] = 0xcc
data[2] = 0xcc

What I have tried:

I can't figure out what's wrong...

Posted 25-Feb-19 2:46am

Member 14131869

Updated 25-Feb-19 21:56pm

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2 solutions

Solution 2

Thanks for the quick response,
I had the hexadecimal enabled. I disabled it and it worked

Posted 25-Feb-19 21:56pm

Member 14131869

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Richard Deeming 26-Feb-19 11:05am

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Rick York · Accepted Answer · 2019-02-25T05:01:00

Solution 1

I think the second parameter is wrong. Here is the prototype of the function:

C++

void glReadPixels( GLint x, 
       GLint y, 
       GLsizei width, 
       GLsizei height, 
       GLenum format, 
       GLenum type, 
       GLvoid* data );

The first two parameters are the location to begin reading. The second parameter, y, should probably be 0 also but certainly not winHeight because that would be an invalid value. The y parameter can be winHeight-1 at most.

Posted 25-Feb-19 5:01am

Rick York

Comments

Member 14131869 25-Feb-19 13:04pm

//LastX and LastY is the mouse position
glReadPixels(LastX, winHeight - 1 - LastY, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, data);

but still it doesn't return valid values.

Rick York 25-Feb-19 13:41pm

Note that you are using GL_RGBA so you need four bytes to hold the data for one pixel with that argument.

I tried this in my little app I call GLplayground with this code :Hide Copy Code

void COpenGLView::OnLButtonDown( UINT nFlags, CPoint point )
{
    trace( _T( "OnLButtonDown at %d,%d - size is %d,%d\n" ),
        point.x, point.y, m_WinSize.cx, m_WinSize.cy );
    int ycoord = m_WinSize.cy - 1 - point.y;
    UCHAR data[4] = { 0 };
    glReadPixels( point.x, ycoord, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, data );
    traceDump( data, 4 );
    __super::OnLButtonDown( nFlags, point );
}

and it worked and displayed values that appear to be correct. I recommend trying something similar. FWIW, this is an MFC app on windows. There are other sample apps at this site you can try this with if you want to. This code should be usable in any mouse click handler function in another framework except you will probably need to replace the trace and traceDump calls with something usable for you. The traceDump function just display hex output so a printf with a format string of %02X for each byte of the pixel will work the same.