Du Fu's seemingly negative commentary on the prized horse paintings of Han Gan ignited a controversy that has persisted to the present day. The tenor of his work changed as he developed his style and adapted to his surroundings ("chameleon-like" according to Watson): his earliest works are in a relatively derivative, courtly style, but he came into his own in the years of the rebellion. Owen comments on the "grim simplicity" of the Qinzhou poems, which mirrors the desert landscape; the works from his Chengdu period are "light, often finely observed"; while the poems from the late Kuizhou period have a "density and power of vision". Although he wrote in all poetic forms, Du Fu is best known for his lǜshi, a type of poem with strict constraints on form and content, for example: About two-thirds of Du Fu's 1500 extant works are in this form, and he is generally considered to be its leading exponent. His best lǜshi use the parallelisms required by the form to add expressive content rather than as mere technical restrictions. Hawkes comments that "it is amazing that Tu Fu is able to use so immensely stylized a form in so natural a manner". Influence According to the Encyclopædia Britannica, Du Fu's writings are considered by many literary critics to be among the greatest of all time, and it states "his dense, compressed language makes use of all the connotative overtones of a phrase and of all the intonational potentials of the individual word, qualities that no translation can ever reveal." In his lifetime and immediately following his death, Du Fu was not greatly appreciated. In part this can be attributed to his stylistic and formal innovations, some of which are still "considered extremely daring and bizarre by Chinese critics." There are few contemporary references to him—only eleven poems from six writers—and these describe him in terms of affection, but not as a paragon of poetic or moral ideals. Du Fu is also poorly represented in contemporary anthologies of poetry. However, as Hung notes, he "is the only Chinese poet whose influence grew with time", and his works began to increase in popularity in the ninth century. Early positive comments came from Bai Juyi, who praised the moral sentiments of some of Du Fu's works (although he found these in only a small fraction of the poems), and from Han Yu, who wrote a piece