# High-Resolution Hologram Calculation Method Based on Light Field Image Rendering
## 4 Discussion  
Encoding the high-resolution light field image into a high-resolution hologram has two important advantages: displaying 3D images with higher resolutions, and dramatically decreased calculation times of high-resolution holograms. In our proposed method, a pinhole array behind the holographic plane is used for the EI's rendering, and a divergent sphere wave is used as the phase information for each EI. However, the pinhole array can also be set in front of the holographic plane, and the convergent sphere wave can be used as phase information.  
In the calculation, converting each EI to the complex amplitude information on each hogel plane using FFT is not needed in the proposed method. Additionally, the phase factor of divergent sphere waves is the same for all hogels, which means the phase factor only needs to be calculated one time and can be reused for each hogel calculation. This direct encoding method can greatly reduce the computational time of high-resolution holograms.  
In the encoding process of holograms, a plane wave is used as a reference wave for simplicity. Actually, a divergent sphere wave is more practical, because the white light we used (or a white light LED) for illumination of the hologram is approximately a divergent wave. The reason why a white light can be used to reconstruct the hologram is that there is minimal distance between the reconstructed image and the holographic plane, similar to the case of the image holography, the color dispersion due to the white light illumination does not have a large influence on the reproduced 3D image. It should be noted that to display the hologram with laser as illumination is still possible with a higher resolution of reconstructed 3D images, but speckle noise will exist.  
We demonstrate two experiments for 3D display with different hogel size and sampling pitch. From the results, we can conclude that the calculation time is associated with the number of object points in the 3D model and EIs, as well as the sampling interval in each EI. Well-designed parameters will guarantee high-quality reconstructed images with a relative short computational time. Parallel computing using a GPU (graphics processing unit) is still possible for further reduction of computational time.  
Figure 8: (**a–c**) Three reconstructed images from different viewpoints. Video S2 shows the reconstruction from different viewpoints.  
The displayed results with white light as illumination are a grayscale 3D display. However, the concept of full color rainbow holography [9; 12] can also be applied to our method for a full color 3D display at the cost of losing vertical parallax.  
In the proposed method, the resolution of each EI and the size of the EI or hogel are related, which means a large FOV will cause large hogel sizes and lead to more of the fence effect. A large-depth 3D display is not possible with this method because an image far from the holographic plane will be blurred due to the white light illumination. However, the proposed method is still valuable for holographic packing or 3D display purposes with relatively small depth.