liste du blend mode
bm_zero: Blend factor is (0, 0, 0, 0).
bm_one: Blend factor is (1, 1, 1, 1).
bm_src_color: Blend factor is (Rs, Gs, Bs, As).
bm_inv_src_color: Blend factor is (1–Rs, 1–Gs, 1–Bs, 1–As).
bm_src_alpha: Blend factor is (As, As, As, As).
bm_inv_src_alpha: Blend factor is (1–As, 1–As, 1–As, 1–As).
bm_dest_alpha: Blend factor is (Ad, Ad, Ad, Ad).
bm_inv_dest_alpha: Blend factor is (1–Ad, 1–Ad, 1–Ad, 1–Ad).
bm_dest_color: Blend factor is (Rd, Gd, Bd, Ad).
bm_inv_dest_color: Blend factor is (1–Rd, 1–Gd, 1–Bd, 1–Ad).
bm_src_alpha_sat: Blend factor is (f, f, f, 1); f = min(As, 1–Ad).
pas de subtract.