Render Passes in "Seal Guardian"

Introduction
"Seal Guardian" uses a forward renderer to render the scene. Because we need to support mobile platform, we don't have too many effect in it. But still it consists of a few render passes to compose an image.

Shadow Map Pass
To calculate dynamic shadow of the scene, we need to render the depth of the meshes from the light point of view. We render them into a 1024x1024 shadow map.
Standard shadow map

Then we use the Exponential Shadow Map method to blur the shadow map into a 512x512 shadow map.
ESM blurred shadow map

(Note that this pass may be skipped according to current performance setting.)

Opaque Geometry Pass
In this pass, we render the scene meshes into a RGBA8 render target. We compute all the lighting including direct lighting, indirect lighting(lightmap or SH probe), tone mapping in this single pass. This is because on iOS, reducing render pass may have a better performance, so we choose to combine all the calculation into a single pass.
Tonemapped opaque scene color
Opaque geometry depth bufer

To reduce the impact of overdraw, we pre-compute a visibility set to avoid drawing occluded mesh (may talk about it in future post). Also we want to add a bloom pass to enhance the effect of bright pixels, we compute a bloom value in this pass according to the pre-tone mapped value and store it in the alpha channel of this pass.

Transparent Geometry Pass
In this pass, we render transparent mesh and particle. We blend the post-tonemapped color with the opaque geometry due to performance reason. Also, because we store the bloom intensity in the alpha channel and we want the alpha geometry to affect the bloom result. We solve this by 2 different methods depending on the game runs on which platform.

On iOS, we render the mesh directly to the render target of the opaque geometry pass with a shader similar to the opaque pass by outputting tonemapped  scene color in RGB and bloom intensity in A. To blend those 4 values over the opaque value, we use the EXT_shader_framebuffer_fetch OpenGL extension. So the blending happens at the end of the transparent geometry shader and we choose the simple blending formula below by using the opacity of the mesh(because we want to make it consistent with other platform):
RGB= mesh color * mesh alpha + dest color * (1 - mesh alpha)
A = mesh bloom intensity
* mesh alpha + dest bloom intensity * (1 - mesh alpha)
On Windows and Mac, the EXT_shader_framebuffer_fetch does not exist. We render all the transparent meshes into a separate RGBA8 render target. We compute the scene color and bloom intensity similar to opaque pass, but before writing to the render target, we decompose the RGB scene color into luma and chroma and store the chroma value in checkerboard pattern similar to this paper(slide 104). So we can store luma+chroma in RG channel, bloom intensity in B channel and opacity of mesh in the A channel of the render target.
Transparent render target on Windows platform

Finally, we can blend this transparent texture over the opaque geometry pass render target.
Composed opaque and transform geometry

Post Process Pass
After those geometry passes, we can blend in the bloom filter. We make several blur passes for those bright pixels and additive blend over the previous render pass output to enhance the bright effect.
Blurred bright pixels
Additive blended bloom texture with scene color

Then we compute a simplified(but not very accurate, due to the lack of a velocity buffer) temporal anti-aliasing using the color and depth buffer of current frame and previous 2 frames. One thing we didn't mention is that, during rendering the opaque and transparent meshes, we jitter the camera projection by half a pixel, alternating between odd and even frame, similar to the figure below, so that we can have sub-pixel information for anti-aliasing.
Temporal AA jitter pattern
Temporal anti-aliased image

Conclusion
In this post, we break down the render passes in "Seal Guardian", which compose of mainly 4 parts: shadow map, opaque geometry, transparent geometry and post process passes. By making less render pass, we can achieve a constant 60FPS in most cases (if target framerate is not met, we may skip some render pass such as temporal AA and shadow).

Lastly, "Seal Guardian" has already been released on Steam / Mac App Store / iOS App Store. If you want to support us to develop games with custom tech, then buying a copy of the game on any platform will help. Thank you very much.

References
[1] The Art and Technology behind Crysis 3 http://www.crytek.com/download/fmx2013_c3_art_tech_donzallaz_sousa.pdf

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