Image-based data, such as a block of texel data, is accessed. The data includes sets of color component values. A luminance value is computed for each set of color components values, generating a range of luminance values. A first set and a second set of color component values that correspond to the minimum and maximum luminance values are selected from the sets of color component values. A third set of color component values can be mapped to an index that identifies how the color component values of the third set can be decoded using the color component values of the first and second sets. The index value is selected by determining where the luminance value for the third set lies in the range of luminance values.