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69 lines
3.1 KiB
Go
69 lines
3.1 KiB
Go
// Copyright 2017 The Ebiten Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Package restorable offers an Image struct that stores image commands
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// and restores its pixel data from the commands when context lost happens.
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//
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// When a function like DrawImage or Fill is called, an Image tries to record
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// the information for restoring.
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//
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// * Context lost
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//
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// Contest lost is a process that information on GPU memory are removed by OS
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// to make more room on GPU memory.
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// This can happen e.g. when GPU memory usage is high, or just switching applications
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// might cause context lost on mobiles.
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// As Ebitengine's image data is on GPU memory, the game can't continue when context lost happens
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// without restoring image information.
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// The package restorable is the package to record information for such restoring.
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//
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// * DrawImage
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//
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// DrawImage function tries to record an item of 'draw image history' in the target image.
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// If a target image is stale or volatile, no item is created.
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// If an item of the history is created,
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// it can be said that the target image depends on the source image.
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// In other words, If A.DrawImage(B, ...) is called,
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// it can be said that the image A depends on the image B.
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//
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// * Fill, WritePixels and Dispose
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//
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// These functions are also drawing functions and the target image stores the pixel data
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// instead of draw image history items. There is no dependency here.
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//
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// * Making images stale
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//
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// After any of the drawing functions is called, the target image can't be depended on by
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// any other images. For example, if an image A depends on an image B, and B is changed
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// by a Fill call after that, the image A can't depend on the image B anymore.
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// In this case, as the image B can no longer be used to restore the image A,
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// the image A becomes 'stale'.
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// As all the stale images are resolved before context lost happens,
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// draw image history items are kept as they are
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// (even if an image C depends on the stale image A, it is still fine).
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//
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// * Stale image
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//
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// A stale image is an image that can't be restored from the recorded information.
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// All stale images must be resolved by reading pixels from GPU before the frame ends.
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// If a source image of DrawImage is a stale image, the target always becomes stale.
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//
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// * Volatile image
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//
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// A volatile image is a special image that is always cleared when a frame starts.
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// For instance, the game screen passed via the update function is a volatile image.
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// A volatile image doesn't have to record the drawing history.
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// If a source image of DrawImage is a volatile image, the target always becomes stale.
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package restorable
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