Rethinking the Inception Architecture for Computer Vision Christian Szegedy Vincent Vanhoucke Sergey Ioffe Jon Shlens Google Inc. [email protected] [email protected] [email protected] [email protected] Zbigniew Wojna University College London [email protected] Abstract networks. VGGNet [18] and GoogLeNet [20] yielded simi- larly high performance in the 2014 ILSVRC [16] classifica- Convolutional networks are at the core of most state- tion challenge. One interesting observation was that gains of-the-art computer vision solutions for a wide variety of in the classification performance tend to transfer to signifi- tasks. Since 2014 very deep convolutional networks started cant quality gains in a wide variety of application domains. to become mainstream, yielding substantial gains in vari- This means that architectural improvements in deep con- ous benchmarks. Although increased model size and com- volutional architecture can be utilized for improving perfor- putational cost tend to translate to immediate quality gains mance for most other computer vision tasks that are increas- for most tasks (as long as enough labeled data is provided ingly reliant on high quality, learned visual features. Also, for training), computational efficiency and low parameter improvements in the network quality resulted in new appli- count are still enabling factors for various use cases such as cation domains for convolutional networks in cases where mobile vision and big-data scenarios. Here we are explor- AlexNet features could not compete with hand engineered, ing ways to scale up networks in ways that aim at utilizing crafted solutions, e.g. proposal generation in detection[4]. the added computation as efficiently as possible by suitably Although VGGNet [18] has the compelling feature of factorized convolutions and aggressive regularization. We architectural simplicity, this comes at a high cost: evalu- benchmark our methods on the ILSVRC 2012 classification ating the network requires a lot of computation. On the challenge validation set demonstrate substantial gains over other hand, the Inception architecture of GoogLeNet [20] the state of the art: 21.2% top-1 and 5.6% top-5 error for was also designed to perform well even under strict con- single frame evaluation using a network with a computa- straints on memory and computational budget. For ex- tional cost of 5 billion multiply-adds per inference and with ample, GoogleNet employed around 7 million parameters, using less than 25 million parameters. With an ensemble of which represented a 9 reduction with respect to its prede- 4 models and multi-crop evaluation, we report 3.5% top-5 cessor AlexNet, which× used 60 million parameters. Further- error and 17.3% top-1 error on the validation set and 3.6% more, VGGNet employed about 3x more parameters than top-5 error on the official test set. AlexNet. The computational cost of Inception is also much lower 1. Introduction than VGGNet or its higher performing successors [6]. This has made it feasible to utilize Inception networks in big-data Since the 2012 ImageNet competition [16] winning en- scenarios[17], [13], where huge amount of data needed to try by Krizhevsky et al [9], their network “AlexNet” has be processed at reasonable cost or scenarios where memory been successfully applied to a larger variety of computer or computational capacity is inherently limited, for example vision tasks, for example to object-detection [5], segmen- in mobile vision settings. It is certainly possible to mitigate tation [12], human pose estimation [22], video classifica- parts of these issues by applying specialized solutions to tar- tion [8], object tracking [23], and superresolution [3]. get memory use [2], [15] or by optimizing the execution of These successes spurred a new line of research that fo- certain operations via computational tricks [10]. However, cused on finding higher performing convolutional neural these methods add extra complexity. Furthermore, these networks. Starting in 2014, the quality of network architec- methods could be applied to optimize the Inception archi- tures significantly improved by utilizing deeper and wider tecture as well, widening the efficiency gap again. 12818 Still, the complexity of the Inception architecture makes ity merely provides a rough estimate of information it more difficult to make changes to the network. If the ar- content. chitecture is scaled up naively, large parts of the computa- tional gains can be immediately lost. Also, [20] does not 2. Higher dimensional representations are easier to pro- provide a clear description about the contributing factors cess locally within a network. Increasing the activa- that lead to the various design decisions of the GoogLeNet tions per tile in a convolutional network allows for architecture. This makes it much harder to adapt it to new more disentangled features. The resulting networks use-cases while maintaining its efficiency. For example, will train faster. if it is deemed necessary to increase the capacity of some 3. Spatial aggregation can be done over lower dimen- Inception-style model, the simple transformation of just sional embeddings without much or any loss in rep- doubling the number of all filter bank sizes will lead to a resentational power. For example, before performing a 4x increase in both computational cost and number of pa- more spread out (e.g. 3 3) convolution, one can re- rameters. This might prove prohibitive or unreasonable in a duce the dimension of the× input representation before lot of practical scenarios, especially if the associated gains the spatial aggregation without expecting serious ad- are modest. In this paper, we start with describing a few verse effects. We hypothesize that the reason for that general principles and optimization ideas that that proved is the strong correlation between adjacent unit results to be useful for scaling up convolution networks in efficient in much less loss of information during dimension re- ways. Although our principles are not limited to Inception- duction, if the outputs are used in a spatial aggrega- type networks, they are easier to observe in that context as tion context. Given that these signals should be easily the generic structure of the Inception style building blocks compressible, the dimension reduction even promotes is flexible enough to incorporate those constraints naturally. faster learning. This is enabled by the generous use of dimensional reduc- tion and parallel structures of the Inception modules which 4. Balance the width and depth of the network. Optimal allows for mitigating the impact of structural changes on performance of the network can be reached by balanc- nearby components. Still, one needs to be cautious about ing the number of filters per stage and the depth of doing so, as some guiding principles should be observed to the network. Increasing both the width and the depth maintain high quality of the models. of the network can contribute to higher quality net- works. However, the optimal improvement for a con- 2. General Design Principles stant amount of computation can be reached if both are increased in parallel. The computational budget should Here we will describe a few design principles based therefore be distributed in a balanced way between the on large-scale experimentation with various architectural depth and width of the network. choices with convolutional networks. At this point, the util- ity of the principles below are speculative and additional fu- Although these principles might make sense, it is not ture experimental evidence will be necessary to assess their straightforward to use them to improve the quality of net- domain of validity. Still, grave deviations from these prin- works out of box. The idea is to use them judiciously in ciples tended to result in deterioration in the quality of the ambiguous situations only. networks and fixing situations where those deviations were detected resulted in improved architectures. 3. Factorizing Convolutions with Large Filter Size 1. Avoid representational bottlenecks, especially early in the network. Feed-forward networks can be repre- Much of the original gains of the GoogLeNet net- sented by an acyclic graph from the input layer(s) to work [20] arise from a very generous use of dimension re- the classifier or regressor. This defines a clear direction duction, just like in the “Network in network” architecture for the information flow. For any cut separating the in- by Lin et al [?]. This can be viewed as a special case of fac- puts from the outputs, one can access the amount of torizing convolutions in a computationally efficient manner. information passing though the cut. One should avoid Consider for example the case of a 1 1 convolutional layer bottlenecks with extreme compression. In general the followed by a 3 3 convolutional× layer. In a vision net- representation size should gently decrease from the in- work, it is expected× that the outputs of near-by activations puts to the outputs before reaching the final represen- are highly correlated. Therefore, we can expect that their tation used for the task at hand. Theoretically, infor- activations can be reduced before aggregation and that this mation content can not be assessed merely by the di- should result in similarly expressive local representations. mensionality of the representation as it discards impor- Here we explore other ways of factorizing convolutions tant factors like correlation structure; the dimensional- in various settings, especially in order to increase the com- 2819 Factorization with Linear vs ReLU activation 0.8 0.7 ReLU Linear 0.6 0.5 0.4 Top−1 Accuracy 0.3 0.2 0.1 0 0 0.5 1 1.5 2 2.5 3 3.5 4 6 Iteration x 10 Figure 1. Mini-network replacing the 5 × 5 convolutions. Figure 2. One of several control experiments between two Incep- tion models, one of them uses factorization into linear + ReLU layers, the other uses two ReLU layers.