There are two losses used in Stage-I of StackGAN as follows:

• Generator loss
• Discriminator loss

The discriminator loss  can be represented as follows:

The preceding equation is pretty self-explanatory. It represents the loss function for the discriminator network, in which both networks are conditioned on the text embeddings. 

The generator loss  can be represented as follows:

The preceding equation is pretty self-explanatory too. It represents the loss function for the generator network, in which both networks are conditioned on the text embeddings. Also, it includes a KL-divergence term to the loss function.