Open Vocabulary Semantic Scene Sketch Understanding

We study the underexplored but fundamental problem of machine understanding of abstract freehand scene sketches. We introduce a sketch encoder that ensures a semanticallyaware feature space, which we evaluate by testing its performance on a semantic sketch segmentation task. To train our model, we rely only on bitmap sketches accompanied by brief captions, avoiding the need for pixel-level annotations. To generalize to a large set of sketches and categories, we build upon a vision transformer encoder pretrained with the CLIP model. We freeze the text encoder and perform visual-prompt tuning of the visual encoder branch while introducing a set of critical modifications. First, we augment the classical key-query (k-q) self-attention blocks with value-value (v-v) self-attention blocks. Central to our model is a two-level hierarchical training that enables efficient semantic disentanglement: The first level ensures holistic scene sketch encoding, and the second level focuses on individual categories. In the second level of the hierarchy, we introduce cross-attention between the text and vision branches. Our method outperforms zero-shot CLIP segmentation results by 37 points, reaching a pixel accuracy of 85.5% on the FS-COCO sketch dataset. Finally, we conduct a user study that allows us to identify further improvements needed over our method to reconcile machine and human understanding of freehand scene sketches.