Write text to transparent GIF

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• Download sources (VS 2005) - 170.42 KB

Abstract

This article describes how to edit a transparent GIF image by writing a text on it and save it back to disk without loosing the transparency of the image.

You can also edit the image by appending other images to it and also by drawing shapes, but it is not included in this article and I may add it later.

Introduction

As GIF images are indexed (had a color palette and pixel color is represented by the index of the color in the palette), you cannot create Graphics object from the Image object containing the GIF image.

If you tried to get the Graphics object by executing

Graphics g = Graphics.FromImage(gifImage);

You will recieve an exception with the following message:

"A Graphics object cannot be created from an image that has an indexed pixel format."

So you cannot edit the GIF image directly and if you converted it to a non-indexed image then try to write it back to the dist, you will loose the transparency information.

Approach

The approach is to convert the GIF into non-indexed image so all the modifications required can be done through its Graphics object, then convert it back into an indexed image to be saved as GIF.

It is easy to convert the indexed image to non-indexed image by simply creating a new non-indexed image with the same width/height of the original image then draw the original image to it.

//Get the Graphics object of the image to use in drawing.
using (Graphics g = Graphics.FromImage(this.DestinationImage))
{
    //Draw the original image.
    g.DrawImage(this.SourceImage, 0, 0);
    //Free the resources.
    g.Dispose();
}

As the image now is non-indexed image we can easily draw shapes, images and text to it using the Graphics object.

g.TextRenderingHint = TextRenderingHint.AntiAlias;
g.DrawString(title, titleFont, titleBrush, left, top, StringFormat.GenericTypographic);

The problem now is how to convert the non-indexed image back into an indexed image, the same approach used to convert the indexed into non-indexed will be used. We will create a new indexed image with the same width/height.

//Create an indexed image.
Bitmap dest = new Bitmap(src.Width, src.Height, PixelFormat.Format8bppIndexed);

We should now draw the non-indexed image content to the indexed image, but before doing that where we get the palette that will be used in the resulting image? For now I used the palette from the original image, but you can modify the code to deduce the palette from the non-indexed image.  

//Set the palette of the image.
dest.Palette = palette;

We should now obtain the transparent color index and cache the palette into a dictionary to speed up the search for color.

//Create a dictionary of colors to speed up the search.
Dictionary colors = new Dictionary();
//The transparent color index.
int transparent = 255;
//Load the dictionary with the given palette.
for (int i = 0; i < palette.Entries.Length; i++)
{
    colors[Color2Int(palette.Entries[i])] = i;
    if (palette.Entries[i].A == 0)
        transparent = i;
}

As we are not able to use the graphics object of the indexed image so we should modify the image raw data. To be able to obtain the image raw data you should first lock it for modification.

Rectangle rect = new Rectangle(0, 0, src.Width, src.Height);
//Lock the image data so you can modify it.
BitmapData destData = dest.LockBits(rect, ImageLockMode.ReadWrite, dest.PixelFormat);

We should obtain the raw data of the image so we can edit it.

//The number of bytes in each scan line (row in image) of the image.
int dStride = Convert.ToInt32(Math.Abs(destData.Stride));
//Create a buffer to hold the image data.
byte[] destBytes = new byte[dest.Height * dStride];
//Copy the image data into the buffer.
IntPtr destPtr = destData.Scan0;
Marshal.Copy(destPtr, destBytes, 0, dest.Height * dStride);

We should loop for each pixel in the non-indexed image, get the pixel image and obtain the index of the pixel color from the palette.

If the pixel color is transparent (Alpha part is zero), we use the transparent index. If the pixel color found in the palette we use this index.

If the pixel color is not found in the palette, then we choose the nearest color. The nearest color in the palette have the minimum distance from the original color based on the formula:
distance = (o.Red-d.Red)2 + (o.Blue-d.Blue)2 + (o.Green-d.Green)2

//Get the color of the pixel.
Color c = src.GetPixel(col, row);
int index = 255;
if (c.A == 0) //Transparent
{
    index = transparent;
}
else
{
    //Get the nearst color from the palette.
    int ic = Color2Int(c);
    if (colors.ContainsKey(ic))
    {
        index = colors[ic];
    }
    else
    {
        index = GetNearestColor(palette, c);
        colors[ic] = index;
    }
}

Update the image raw data with the selected color.

destBytes[row * dStride + col] = (byte)index;

After updating all image data, we should copy data back to the image and unlock the data.

//Copy the image data back to the image.
Marshal.Copy(destBytes, 0, destPtr, dest.Height * dStride);
//Unlock the data.
dest.UnlockBits(destData);

Using the code

When the user presses the Open button, we simply load the image into SourceImage, DestinationImage.

this.SourceImage = new Bitmap(fileName);
this.DestinationImage = this.SourceImage;

The image is now loaded and stored in SourceImage. When the user presses the Write button we should create a new non-indexed image from the source image and prepare the fonts and brushes for the title and text body.

//Create a blank image.
this.DestinationImage = new Bitmap(this.SourceImage.Width, this.SourceImage.Height, PixelFormat.Format32bppArgb);
//The font used in writing the title.
Font titleFont = new Font("Arial Black", 36, FontStyle.Italic, GraphicsUnit.Point);
//The brush used in writing the title.
Brush titleBrush = new SolidBrush(titleColor);
//The font used in writing the text.
Font bodyFont = new Font("Arial", 24, FontStyle.Bold | FontStyle.Italic, GraphicsUnit.Point);
//The brush used in writing the text.
Brush bodyBrush = new SolidBrush(bodyColor);

As the image is now non-index image we can get its Graphics object and use all the drawing methods exposed by the Graphics object.

//Get the Graphics object of the image to use in drawing.
using (Graphics g = Graphics.FromImage(this.DestinationImage))
{
    //Draw the original image.
    g.DrawImage(this.SourceImage, 0, 0);
    //Used to write smooth text.
    g.TextRenderingHint = TextRenderingHint.AntiAlias;
    //Write the text you want.
    g.DrawString(title, titleFont, titleBrush, left, top, StringFormat.GenericTypographic);

    float y = top + g.MeasureString(title, titleFont).Height;
    g.DrawString(body, bodyFont, bodyBrush, left, y, StringFormat.GenericTypographic);

    //Free the resources.
    g.Dispose();
}

Now the edited image is stored in DestinationImage but it is non-indexed image. When the user presses the Save As button we should convert it into an indexed image and save it to disk.

Bitmap gif = CreateIndexedImage(this.DestinationImage, this.SourceImage.Palette);
gif.Save(fileName, ImageFormat.Gif);