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116 lines
4.5 KiB
ReStructuredText
116 lines
4.5 KiB
ReStructuredText
########################
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Sky System and Structure
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########################
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Overview
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********
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The sky system is made from multiple components that contribute to the final result.
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1. Background colour and fog
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2. Atmosphere
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3. Clouds
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4. Stars
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5. Sun
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6. Moons
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7. Weather effects
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.. image:: https://gitlab.com/OpenMW/openmw-docs/-/raw/master/docs/source/reference/modding/_static/sky-system-overview.png
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:align: center
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1. Background Colour and Fog
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****************************
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Even when nothing else is being rendered, OpenMW will always show the basic
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background colour. The distant fog effect is based off this colour as well.
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Other elements of the sky are rendered on top of the background to compose the
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sky. The colour of the background changes depending on the time of day and
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current weather.
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2. Atmosphere
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*************
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A mesh that contributes the blue colour of the sky. It is a rough cylinder in
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shape without the bottom face and with the normals pointing inwards. During the
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day, it is light blue and transitions to a very dark blue, almost black, during
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the night.
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Towards the bottom edge the mesh gradually becomes transparent and blends with
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the background colour. Transparency is done per vertex and OpenMW decides which
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vertices are transparent based on their index. This adds a requirement for a
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very strict vertex and face order on this mesh to blend properly.
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.. image:: https://gitlab.com/OpenMW/openmw-docs/-/raw/master/docs/source/reference/modding/_static/sky-system-mesh-atmosphere.jpg
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:align: center
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3. Clouds
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*********
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A mesh that renders a tiling, scrolling texture of the clouds. It is a flat dome
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in shape with the normals pointing inwards. Towards the boundary edge the mesh
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becomes transparent and blends with the background colour. As with the
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atmosphere, there is a very strict vertex order on this mesh to blend properly.
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.. image:: https://gitlab.com/OpenMW/openmw-docs/-/raw/master/docs/source/reference/modding/_static/sky-system-mesh-clouds.jpg
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:align: center
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By default, the UVs of this mesh are scaled so the clouds texture repeats five
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times across the sky. The weather system blends between different cloud textures
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depending on the current weather. Speed of the clouds moving across the sky can
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be set per weather type.
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4. Stars
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********
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A dome shaped mesh that shows the stars during the night. It starts to become
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visible during sunset and goes back to transparent during sunrise. At its bottom
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edge it blends to transparency which is defined with the vertex colour. White is
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full opacity while black is full transparency. The mesh ends above the horizon
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to prevent the stars being visible near the horizon when underwater.
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.. image:: https://gitlab.com/OpenMW/openmw-docs/-/raw/master/docs/source/reference/modding/_static/sky-system-mesh-stars.jpg
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:align: center
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It can use multiple textures to show different star constellations. In addition,
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extra meshes can be used to blend nebulae across the sky.
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5. Sun
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******
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The sun is a billboard that moves across the sky and is composed of two texures.
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One shows the regular sun sphere, the other is the sun glare that is added on
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top of the sun sphere. Glare strength adjusts dynamically depending on how
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obstructed the view to the sun is.
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6. Moons
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********
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Each moon is its own billboard moving across the sky and both moons are rendered
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the same way. First, a circle texture ``tx_mooncircle_full_m.dds`` or
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``tx_mooncircle_full_s.dds`` is used to mask the background and this mask stays
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the same for all moon phases. A texture with actual moon graphics is then drawn
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on top of the mask. Depending on the current moon phase, the appropriate variant
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of the moon texture is used. The texture on top is additively blended so any
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area we wish to show as transparent is achieved with black pixels. The following
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image shows all the separate textures needed for one moon.
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.. image:: https://gitlab.com/OpenMW/openmw-docs/-/raw/master/docs/source/reference/modding/_static/sky-system-moon-textures.jpg
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:align: center
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7. Weather Effects
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******************
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These are particle emitters used to display weather phenomena such as rain,
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snow, or dust clouds. Originally, Morrowind used .nif files with a number of
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planes and baked animations. In OpenMW, these effects are done through code and
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are currently hardcoded. The particle emmitters emit particles around the player
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in a big enough area so it looks like the whole world is affected by the
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weather.
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Settings
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********
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Colour and other settings for each weather type can be edited in ``openmw.cfg``
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configuration file.
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