How can I compare the RGB value in pixel?

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?

Use glReadPixels instead.

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?

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

You can read data from a texture using glGetTexImage.