设置帧延迟后保存动画 GIF 图像

Question

I want to modify the frame-delay in animated GIF files using the PropertyItems in System.Drawing.Image. This works well, but I'm not able to save the changes to file.

Thx in advance, Peter

Win10 x64; .NET Framework 4.6.2

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Windows.Forms;
using System.Drawing;
using System.IO;
using System.Linq;

/// <summary>
/// Change delay and save to file.
/// </summary>
/// <param name="gifImage">the input-gif-image</param>
/// <param name="newDelay">new delay in 1/100 sec.</param>
/// <param name="fileToSave">full path to save the changed image</param>
private static void ChangeGifFrameDelayAndSave(Image gifImage, int newDelay, string fileToSave)
{
    // convert newDelay to raw bytes
    byte[] frameDelayBytes = BitConverter.GetBytes(newDelay);   //4 Bytes

    var frameDimension = new System.Drawing.Imaging.FrameDimension(gifImage.FrameDimensionsList[0]);
    int frameCount = gifImage.GetFrameCount(frameDimension);
    var framePropItem = gifImage.GetPropertyItem(0x5100);       //get propItem with frame-delays
    byte[] delayRawData = framePropItem.Value;

    // change delays in each frame
    for (int frameIndex = 0; frameIndex < frameCount; frameIndex++)
    {
        delayRawData[frameIndex * 4 + 0] = frameDelayBytes[0];
        delayRawData[frameIndex * 4 + 1] = frameDelayBytes[1];
        delayRawData[frameIndex * 4 + 2] = frameDelayBytes[2];
        delayRawData[frameIndex * 4 + 3] = frameDelayBytes[3];
    }

    // set propItem back to the image
    framePropItem.Value = delayRawData;
    gifImage.SetPropertyItem(framePropItem);

    // test only:
    {
        PictureBox1.Image = gifImage;  // THIS WORKS WELL: newDelay is applied to the image and I can see the change
    }

    // now save the Gif to file...
    gifImage.Save(fileToSave, System.Drawing.Imaging.ImageFormat.Gif);

    // unfortunately newDelay is NOT APPLIED to the file!
    // what I'm doing wrong :-(
}

Answer 1

After Microsoft apparently didn't find it necessary to implement the GDI+ components in a presentable way, I have solved the problem now completely differently and low-level.

Thanks to Bjørn for this excellent solution (it's a complete low-level Gif-Editor with UI): https://www.codeproject.com/Articles/1042433/Manipulating-GIF-Color-Tables?msg=5810217#xx5810217xx