使用 dotimage 调整动画 gif 的大小

Question

我有一段代码可以调整动画 gif 的大小。 如果它有帮助，代码将始终将图像调整为更小的尺寸。 （暂时不需要做大）

我正在使用 Atalasoft 的 dotimage 库及其示例代码来进行实际的重采样。 该代码应该从磁盘读取动画 gif，遍历帧并将每个帧的大小调整为新的大小。 当 gif 动画包含相同大小的帧但使用不同大小的帧调整动画的大小时，这可以正常工作（调整大小后帧不会正确地相互重叠），我认为这是因为代码没有计算新的偏移量正确。

我认为是这行代码没有正确计算偏移量： 点point = new Point((int)(frame.Location.X * ratio), (int)(frame.Location.Y * ratio));

这是完整的调整大小例程：

    static private void GenerateGifImage(FileStream fileStream, int OutputWidth, int OutputHeight)
    {            
        // MemoryStream InputStream = new MemoryStream();
        FileStream InputStream = fileStream;
        // fileStream.Write(InputStream.GetBuffer(), 0, (int)InputStream.Position);
        // InputStream.Seek(0, SeekOrigin.Begin);
        Image InputImage = Image.FromStream(InputStream, true, false);

        // this will invalidate the underlying image object in InputImage but the class properties 
        // will still accessible until the object is disposed
        InputStream.Seek(0, SeekOrigin.Begin);

        ImageInfo imageInfo = RegisteredDecoders.GetImageInfo(InputStream);
        InputStream.Seek(0, SeekOrigin.Begin);

        GifDecoder gifDecoder = new GifDecoder();
        int count = gifDecoder.GetFrameCount(InputStream);

        GifFrameCollection gifFrameCollection = new GifFrameCollection();
        gifFrameCollection.Height = OutputHeight;
        gifFrameCollection.Width = OutputWidth;
        // gifFrameCollection.Height = gifDecoder.Frames.Height;
        // gifFrameCollection.Width = gifDecoder.Frames.Width;

        double ratio;
        if (InputImage.Height > InputImage.Width)
        {
            ratio = (double)OutputHeight / (double)InputImage.Height;
        }
        else
        {
            ratio = (double)OutputWidth / (double)InputImage.Width;
        }

        for (int i = 0; i < count; i++)
        {
            GifFrame frame = gifDecoder.Frames[i];

            Rectangle rectangle = new Rectangle(Point.Empty, frame.Image.Size);

            int frameWidth = (int)(frame.Image.Width * ratio);
            int frameHeight = (int)(frame.Image.Height * ratio);

            // account for erratic rounding, seems illogical but has happened earlier when using floats instead of doubles 
            if (frameWidth > OutputWidth)
            {
                frameWidth = OutputWidth;
            }
            if (frameHeight > OutputHeight)
            {
                frameHeight = OutputHeight;
            }

            Size size = new Size(frameWidth, frameHeight);
            // only resize if we have a measureable dimension
            if (size.Width > 0 && size.Height > 0)
            {
                // ResampleCommand resampleCommand = new ResampleCommand(rectangle, size, ResampleMethod.NearestNeighbor);
                ResampleCommand resampleCommand = new ResampleCommand(rectangle, size, ResampleMethod.NearestNeighbor);
                AtalaImage atalaImage = resampleCommand.Apply(frame.Image).Image;
                // save the image for debugging
                // atalaImage.Save("frame" + i.ToString() + ".gif", ImageType.Gif, null);
                // frame.Image.Save("frame-orig" + i.ToString() + ".gif", ImageType.Gif, null);

                // AtalaImage atalaImage = frame.Image;
                Point point = new Point((int)(frame.Location.X * ratio), (int)(frame.Location.Y * ratio));
                // Point point = new Point((int)(frame.Location.X), (int)(frame.Location.Y));
                gifFrameCollection.Add(new GifFrame(atalaImage, point, frame.DelayTime, frame.Interlaced, frame.FrameDisposal, frame.TransparentIndex, frame.UseLocalPalette));
            }
        }
        FileStream saveStream = new FileStream("resized.gif", FileMode.Create, FileAccess.Write, FileShare.Write);
        GifEncoder gifSave = new GifEncoder();
        gifSave.Save(saveStream, gifFrameCollection, null);
        saveStream.Close();
    }

Answer 1

我在 Atalasoft 工作

我对此进行了研究——您的代码绝对正确，并且可以在大小不等的帧上正常工作。你计算的点是正确的。

问题在于，在您的 3 帧 GIF 中，您的第二帧和第三帧被精确地覆盖在第一帧之上，并使用非常复杂的透明蒙版通过它们显示第一帧。当您的图像被重新采样为新尺寸时，遮罩可能不再精确 - 因为您将大小调整为宽度和高度上只有一个像素的差异，所以此遮罩无法匹配。

这个问题有几种解决方案

1. 将第 2 帧叠加到第 1 帧上，然后重新采样并改用该图像
2. 执行 #1，然后提取第 2 帧的矩形
3. 使用裁剪而不是重新采样 - 这似乎是最好的，因为它只有 1 个像素。

我为你编写了 #3 代码——效果很好

    static private void GenerateGifImage(FileStream fileStream, int OutputWidth, int OutputHeight)
    {            
        // MemoryStream InputStream = new MemoryStream();
        FileStream InputStream = fileStream;
        // fileStream.Write(InputStream.GetBuffer(), 0, (int)InputStream.Position);
        // InputStream.Seek(0, SeekOrigin.Begin);
        Image InputImage = Image.FromStream(InputStream, true, false);

        // this will invalidate the underlying image object in InputImage but the class properties 
        // will still accessible until the object is disposed
        InputStream.Seek(0, SeekOrigin.Begin);

        ImageInfo imageInfo = RegisteredDecoders.GetImageInfo(InputStream);
        InputStream.Seek(0, SeekOrigin.Begin);

        GifDecoder gifDecoder = new GifDecoder();
        int count = gifDecoder.GetFrameCount(InputStream);

        GifFrameCollection gifFrameCollection = new GifFrameCollection();
        gifFrameCollection.Height = OutputHeight;
        gifFrameCollection.Width = OutputWidth;

        double ratio;
        if (InputImage.Height > InputImage.Width)
        {
            ratio = (double)OutputHeight / (double)InputImage.Height;
        }
        else
        {
            ratio = (double)OutputWidth / (double)InputImage.Width;
        }

        for (int i = 0; i < count; i++)
        {
            GifFrame frame = gifDecoder.Frames[i];

            Rectangle rectangle = new Rectangle(Point.Empty, frame.Image.Size);

            int newframeWidth = frame.Image.Width;
            int newframeHeight = frame.Image.Height;
            if (newframeWidth > OutputWidth || newframeHeight > OutputHeight)
            {
                newframeWidth = (int)(frame.Image.Width * ratio);
                newframeHeight = (int)(frame.Image.Height * ratio);
            }

            // account for erratic rounding, seems illogical but has happened earlier when using floats instead of doubles 
            if (newframeWidth > OutputWidth)
            {
                newframeWidth = OutputWidth;
            }
            if (newframeHeight > OutputHeight)
            {
                newframeHeight = OutputHeight;
            }

            Size size = new Size(newframeWidth, newframeHeight);
            // only resize if we have a measureable dimension
            if (size.Width > 0 && size.Height > 0)
            {
                //ResampleCommand resampleCommand = new ResampleCommand(rectangle, size, ResampleMethod.);
                AtalaImage atalaImage = frame.Image;
                if (newframeWidth != frame.Image.Width || newframeHeight != frame.Image.Height)
                {
                    CropCommand command = new CropCommand(new Rectangle(new Point(0, 0), size));
                    atalaImage = command.Apply(frame.Image).Image;
                }
                // AtalaImage atalaImage = frame.Image;
                Point point = new Point((int)(frame.Location.X), (int)(frame.Location.Y));
                // Point point = new Point((int)(frame.Location.X), (int)(frame.Location.Y));
                gifFrameCollection.Add(new GifFrame(atalaImage, point, frame.DelayTime, frame.Interlaced, frame.FrameDisposal, frame.TransparentIndex, frame.UseLocalPalette));
            }
        }
        FileStream saveStream = new FileStream("resized.gif", FileMode.Create, FileAccess.Write, FileShare.Write);
        GifEncoder gifSave = new GifEncoder();
        gifSave.Save(saveStream, gifFrameCollection, null);
        saveStream.Close();
    }

Answer 2

如果您使用不同的帧大小，则计算出的比率值不正确。您应该计算每个单独帧的比率，以便您关注的行使用正确的比率。我对框架不熟悉，所以不能给你一个准确的例子；但它应该看起来像这样：

GifFrame frame = gifDecoder.Frames[i];
double frameRatio;
if (frame.Height > frame.Width)
{
   frameRatio = (double)OutputHeight / (double)frame.Height;
}
else
{
   frameRatio = (double)OutputWidth / (double)frame.Width;
}

...

Point point = new Point((int)(frame.Location.X * frameRatio), (int)(frame.Location.Y * frameRatio));