When GL_MINMAX is enabled, the RGBA components of incoming pixels are compared to the minimum and maximum values for each component, which are stored in the 2-element minmax table. (The first element stores the minima, and the second element stores the maxima.) If a pixel component is greater than the corresponding component in the maximum element, then the maximum element is updated with the pixel component value. If a pixel component is less than the corresponding component in the minimum element, then the minimum element is updated with the pixel component value. (In both cases, if the internal of the minmax table includes luminance, then the R color component of incoming pixels is used for comparison.) The contents of the minmax table may be retrieved at a later time by calling glGetMinmax. The minmax operation is enabled or disabled by calling glEnable or glDisable, respectively, with an argument of GL_MINMAX.
glMinmax redefines the current minmax table to have entries of the specified by internalformat. The maximum element is initialized with the smallest possible component values, and the minimum element is initialized with the largest possible component values. The values in the previous minmax table, if any, are lost. If sink is GL_TRUE, then pixels are discarded after minmax; no further processing of the pixels takes place, and no drawing, texture loading, or pixel readback will result.
glMinmax is present only if GL_ARB_imaging is returned when glGetString is called with an argument of GL_EXTENSIONS.