How can I compare the RGB value in pixel?

OpenGL OpenGL: Basic Coding
1

AfterI use gluScaleImage(GL_RGB,…GL_UNSIGNED_BYTE,…,pdata) to transfer the pixel address into pdata,how can I compare the RGB value in two different pixel?directly using *pdata or something else?But when I attemp to output *pdata value
,the result is a ‘?’,why?

2

Use glReadPixels instead.

3

but I dont want to display the texture in the
window first,is there any method served to get the pixel RGB value in the input texture directly?

4

Read the data you put when making your texture. There aren’t any other solution to your problem.

5

You can read data from a texture using glGetTexImage.