On image no. 99 identify the percentage of objects that are described as white.
image no. 99 refers to IMG_ID = 99; described as white refers to ATT_CLASS = 'white'; percentage = divide(count(OBJ_SAMPLE_ID) where ATT_CLASS = 'white', count(OBJ_SAMPLE_ID)) as percentage
SELECT CAST(SUM(CASE WHEN T2.ATT_CLASS = 'white' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(OBJ_SAMPLE_ID) FROM IMG_OBJ_ATT AS T1 INNER JOIN ATT_CLASSES AS T2 ON T1.ATT_CLASS_ID = T2.ATT_CLASS_ID WHERE T1.IMG_ID = 99