For creating photorealistic images, Computer Graphics researchers introduced Bidirectional Texture Functions (BTFs), which use view- and illumination-dependent textures for rendering. BTFs require massive storage, and several proposals were made on how to compress them, but very few take into account human perception. We present and discuss an experimental study on how decreasing the texture resolution influences perceived quality of the rendered images. In a visual comparison task, observer quality judgments and gaze data were collected and analysed to determine the optimal downsampling of BTF data without significant loss of their perceived visual quality.