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[[File:Low pixel density vs high pixel density.png|thumb|Figure 1. High Pixel Density Vs Low Pixel Density (image: teknosrc.com)]]

[[File:Low pixel density vs high pixel density.png|thumb|Figure 1. High Pixel Density Vs Low Pixel Density (image: teknosrc.com)]]

Pixel density is the number of pixels in a certain amount of space and it describes the sharpness and clarity for different types of displays (i.e. computers, mobile devices, cameras, etc). Pixel density is often measured in Pixels per Inch (PPI) - the number of pixels present per inch on the display. A display's pixel density is different from its resolution, which is the total number of pixels across the display's width and height (Figure 1). <ref name=”1”>Nowell, P. (2016). Pixel Density, demystified: How pixel density works and how it affects your designs. Retrieved from https://medium.com/@pnowelldesign/pixel-density-demystified-a4db63ba2922</ref> <ref name=”2”>Raghavendar, T.S. (2013). Resolution Vs pixel density in displays – all you need to know. Retrieved from http://teknosrc.com/resolution-vs-pixel-density-in-displays-all-you-need-to-know/</ref> <ref name=”3”>Übergizmo. What is PPI: Pixels per inch, display resolution. Retrieved from http://www.ubergizmo.com/what-is/ppi-pixels-per-inch/</ref>

Pixel density is the number of pixels in a certain amount of space and it describes the sharpness and clarity for different types of displays (i.e. computers, mobile devices, cameras, etc). Pixel density is often measured in Pixels per Inch (PPI) - the number of pixels present per inch on the display. A display's pixel density is different from its resolution, which is the total number of pixels across the display's width and height (Figure 1). <ref name=”1”>Nowell, P. (2016). Pixel Density, demystified: How pixel density works and how it affects your designs. Retrieved from https://medium.com/@pnowelldesign/pixel-density-demystified-a4db63ba2922</ref><ref name=”2”>Raghavendar, T.S. (2013). Resolution Vs pixel density in displays – all you need to know. Retrieved from http://teknosrc.com/resolution-vs-pixel-density-in-displays-all-you-need-to-know/</ref><ref name=”3”>Übergizmo. What is PPI: Pixels per inch, display resolution. Retrieved from http://www.ubergizmo.com/what-is/ppi-pixels-per-inch/</ref>

==Calculating pixels per inch==

==Calculating pixels per inch==

[[File:Relation between pixels per degree and field of view.png|thumb|Figure 2. Relation between pixels per degree and field of view (Image: eenewseurope.com)]]

[[File:Relation between pixels per degree and field of view.png|thumb|Figure 2. Relation between pixels per degree and field of view (Image: eenewseurope.com)]]

When dealing with [[virtual reality]] or [[augmented reality]] (AR) [[head-mounted display|head-mounted displays]] ([[HMD]]s), a more useful metric for pixel density might be Pixels per Degree (PPD). <ref name=”4”>Boger, Y. (2017). Understanding pixel density & retinal resolution, and why it’s important for AR/VR headsets. Retrieved from https://www.roadtovr.com/understanding-pixel-density-retinal-resolution-and-why-its-important-for-vr-and-ar-headsets/</ref> <ref name=”5”>Chinnock, C. (2017). What’s the definition of an immersive display? Retrieved from https://www.insightmedia.info/whats-the-definition-of-an-immersive-display/</ref>

When dealing with [[virtual reality]] or [[augmented reality]] (AR) [[head-mounted display|head-mounted displays]] ([[HMD]]s), a more useful metric for pixel density might be Pixels per Degree (PPD). <ref name=”4”>Boger, Y. (2017). Understanding pixel density & retinal resolution, and why it’s important for AR/VR headsets. Retrieved from https://www.roadtovr.com/understanding-pixel-density-retinal-resolution-and-why-its-important-for-vr-and-ar-headsets/</ref><ref name=”5”>Chinnock, C. (2017). What’s the definition of an immersive display? Retrieved from https://www.insightmedia.info/whats-the-definition-of-an-immersive-display/</ref>

To calculate the HMD’s pixel density in PPD (the number of pixels per degree it presents to the eye), the number of pixels in a horizontal display line has to be divided by the horizontal [[field of view]] provided by the lens. <ref name=”4”></ref>

To calculate the HMD’s pixel density in PPD (the number of pixels per degree it presents to the eye), the number of pixels in a horizontal display line has to be divided by the horizontal [[field of view]] provided by the lens. <ref name=”4”></ref>

Indeed, this is is one of the elements that contribute to the sense of [[immersion]] in VR. An HMD with low pixel density can cause blurring of words and objects, presence of pixelation and the screen door effect. Also, as pixel density increases, lower [[persistence]] is needed to avoid blurring. However, in general terms, a higher PPD number will result in a sharper, more realistic image - one of the factors that influence immersion and [[presence]]. There is an inverse relationship - a trade-off - between PPD and FOV (Figure 2): a narrow FOV results in a higher pixel density and a higher FOV produces a lower pixel density. <ref name=”6”>Chinnock, C. (2017). VR headset design: Time to rethink pixel density vs wider FOV? Retrieved from http://www.eenewseurope.com/design-center/vr-headset-design-time-rethink-pixel-density-vs-wider-fov</ref>

Indeed, this is is one of the elements that contribute to the sense of [[immersion]] in VR. An HMD with low pixel density can cause blurring of words and objects, presence of pixelation and the screen door effect. Also, as pixel density increases, lower [[persistence]] is needed to avoid blurring. However, in general terms, a higher PPD number will result in a sharper, more realistic image - one of the factors that influence immersion and [[presence]]. There is an inverse relationship - a trade-off - between PPD and FOV (Figure 2): a narrow FOV results in a higher pixel density and a higher FOV produces a lower pixel density. <ref name=”6”>Chinnock, C. (2017). VR headset design: Time to rethink pixel density vs wider FOV? Retrieved from http://www.eenewseurope.com/design-center/vr-headset-design-time-rethink-pixel-density-vs-wider-fov</ref>

Chinnock (2017) provides the following example: “If you want a 100-degree horizontal field of view and use a panel with 1080 x 1200 pixels per eye, you get 11 pixels per degree per eye if the optics are perfect and your eye is in the optimal lens-to-eye position. The PPD per eye of the HTC Vive, Oculus CV1 and PlayStation VR headsets are all in the 10 to 15 range today.” A low FOV would result in a higher pixel density, but this could also affect negatively the VR experience since a wider field of view is also important for immersion. <ref name=”6”></ref> <ref name=”7”>VR Lens Lab. Field of view for virtual reality headsets explained. Retrieved from https://vr-lens-lab.com/field-of-view-for-virtual-reality-headsets/</ref>

Chinnock (2017) provides the following example: “If you want a 100-degree horizontal field of view and use a panel with 1080 x 1200 pixels per eye, you get 11 pixels per degree per eye if the optics are perfect and your eye is in the optimal lens-to-eye position. The PPD per eye of the HTC Vive, Oculus CV1 and PlayStation VR headsets are all in the 10 to 15 range today.” A low FOV would result in a higher pixel density, but this could also affect negatively the VR experience since a wider field of view is also important for immersion. <ref name=”6”></ref><ref name=”7”>VR Lens Lab. Field of view for virtual reality headsets explained. Retrieved from https://vr-lens-lab.com/field-of-view-for-virtual-reality-headsets/</ref>

There is still a way to go for VR HMDs to come close to the visual acuity or retinal resolution of people’s eyes. Nevertheless, higher pixel density displays for VR headsets are coming. <ref name=”6”></ref>

There is still a way to go for VR HMDs to come close to the visual acuity or retinal resolution of people’s eyes. Nevertheless, higher pixel density displays for VR headsets are coming. <ref name=”6”></ref>