实现APNG渲染功能

Question

大家好， 目前我正在尝试实现APNG规范，但是在帧渲染方面遇到了一些问题。我的函数如下：

    private void UpdateUI()
    {
        foreach (PictureBox pb in pics)
        {
            APNGBox box = (APNGBox)pb.Tag;
            APNGLib.APNG png = box.png;
            if (box.buffer == null)
            {
                box.buffer = new Bitmap((int)png.Width, (int)png.Height);
            }
            APNGLib.Frame f = png.GetFrame(box.frameNum);
            using (Graphics g = Graphics.FromImage(box.buffer))
            {
                switch (f.DisposeOp)
                {
                    case APNGLib.Frame.DisposeOperation.NONE:
                        break;
                    case APNGLib.Frame.DisposeOperation.BACKGROUND:
                        g.Clear(Color.Transparent);
                        break;
                    case APNGLib.Frame.DisposeOperation.PREVIOUS:
                        if (box.prevBuffer != null)
                        {
                            g.DrawImage(box.prevBuffer, Point.Empty);
                        }
                        else
                        {
                            g.Clear(Color.Transparent);
                        }
                        break;
                    default:
                        break;
                }
                Bitmap read = png.ToBitmap(box.frameNum++);
                switch (f.BlendOp)
                {
                    case APNGLib.Frame.BlendOperation.OVER:
                        g.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceOver;
                        break;
                    case APNGLib.Frame.BlendOperation.SOURCE:
                        g.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceCopy;
                        break;
                    default:
                        break;
                }
                g.DrawImage(read, new Point((int)f.XOffset, (int)f.YOffset));
            }

            box.prevBuffer = box.buffer;
            pb.Image = box.buffer;

            if (box.frameNum >= box.png.FrameCount)
            {
                box.frameNum = 0;
                box.buffer = null;
                box.prevBuffer = null;
            }
        }
    }

The APNGBox is

internal class APNGBox
    {
        public int frameNum = 0;
        public APNGLib.APNG png;
        public Bitmap buffer;
        public Bitmap prevBuffer;
    }

我认为大部分是正确的，因为我已经将其运行在APNG画廊中的所有图像上。但是，其中一些呈现不正确（这个存在伪影问题，而这个不能始终保留左侧的红条）。请注意，您需要在Firefox 3或更高版本中查看动画。

我认为这个问题与我如何处理DISPOSE_OP_PREVIOUS有关，但我想不出我做错了什么。有人能建议我可能错过了什么吗？

- Roadrunner-EX

Answer 1

最终，我成功地解决了问题，并在此发布，以防将来有人遇到类似的问题。事实证明，问题主要有以下三个方面：

1. 如果帧类型为“PREVIOUS”，则不应更改“previous buffer”
2. 应使用上一帧的dispose_op而不是当前帧的dispose_op来确定如何处理
3. 确保仅处置旧帧的区域，而不是整个帧

新代码如下：

    internal class APNGBox
    {
        public int frameNum { get; set; }
        public APNGLib.APNG apng { get; set; }
        public Bitmap buffer { get; set; }
        public Bitmap prevBuffer { get; set; }

        public APNGBox(APNGLib.APNG png)
        {
            frameNum = 0;
            apng = png;
            buffer = apng.ToBitmap(0);
            prevBuffer = null;
        }
    }

    private void UpdateUI()
    {
        foreach (PictureBox pb in pics)
        {
            APNGBox box = (APNGBox)pb.Tag;
            APNGLib.APNG png = box.apng;
            if (!png.IsAnimated)
            {
                if (pb.Image == null)
                {
                    pb.Image = png.ToBitmap();
                }
            }
            else
            {
                if (box.frameNum != png.FrameCount - 1)
                {
                    Bitmap prev = box.prevBuffer == null ? null : new Bitmap(box.prevBuffer);
                    APNGLib.Frame f1 = png.GetFrame(box.frameNum);
                    if (f1.DisposeOp != APNGLib.Frame.DisposeOperation.PREVIOUS)
                    {
                        box.prevBuffer = new Bitmap(box.buffer);
                    }
                    DisposeBuffer(box.buffer, new Rectangle((int)f1.XOffset, (int)f1.YOffset, (int)f1.Width, (int)f1.Height), f1.DisposeOp, prev);
                    box.frameNum++;
                    APNGLib.Frame f2 = png.GetFrame(box.frameNum);
                    RenderNextFrame(box.buffer, new Point((int)f2.XOffset, (int)f2.YOffset), png.ToBitmap(box.frameNum), f2.BlendOp);
                }
                else
                {
                    box.frameNum = 0;
                    box.prevBuffer = null;
                    ClearFrame(box.buffer);
                    RenderNextFrame(box.buffer, Point.Empty, png.ToBitmap(box.frameNum), APNGLib.Frame.BlendOperation.SOURCE);
                }
                pb.Invalidate();
            }
        }
    }

    private void DisposeBuffer(Bitmap buffer, Rectangle region, APNGLib.Frame.DisposeOperation dispose, Bitmap prevBuffer)
    {
        using (Graphics g = Graphics.FromImage(buffer))
        {
            g.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceCopy;

            Brush b = new SolidBrush(Color.Transparent);
            switch (dispose)
            {
                case APNGLib.Frame.DisposeOperation.NONE:
                    break;
                case APNGLib.Frame.DisposeOperation.BACKGROUND:
                    g.FillRectangle(b, region);
                    break;
                case APNGLib.Frame.DisposeOperation.PREVIOUS:
                    if(prevBuffer != null)
                    {
                        g.FillRectangle(b, region);
                        g.DrawImage(prevBuffer, region, region, GraphicsUnit.Pixel);
                    }
                    break;
                default:
                    break;
            }
        }
    }

    private void RenderNextFrame(Bitmap buffer, Point point, Bitmap nextFrame, APNGLib.Frame.BlendOperation blend)
    {
        using(Graphics g = Graphics.FromImage(buffer))
        {
            switch(blend)
            {
                case APNGLib.Frame.BlendOperation.OVER:
                    g.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceOver;
                    break;
                case APNGLib.Frame.BlendOperation.SOURCE:
                    g.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceCopy;
                    break;
                default:
                    break;
            }
            g.DrawImage(nextFrame, point);
        }
    }

    private void ClearFrame(Bitmap buffer)
    {
        using(Graphics g = Graphics.FromImage(buffer))
        {
            g.Clear(Color.Transparent);
        }
    }

- Roadrunner-EX

---

你好，能提供完整的可工作源代码吗？我想在我的WinForms应用程序中显示APNG。谢谢。 - Dede

我已经有一段时间没有处理这些了，但我在这里编写了一些演示：https://github.com/murrple-1/APNGManagement。此外，我相当确定当时我对.net/C#还很陌生，因此依赖项没有NuGet（不过我应该会在某个时候做的）。 - Roadrunner-EX

@dede 刚刚添加了 NuGet 支持。APNGViewer 应用程序对您应该很有用。 - Roadrunner-EX