how to convert 32bpp into 8bpp bitmap

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5.00/5 (1 vote)

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C#

image-processing

I want to convert 32 bpp bitmap into 8 bpp.

If anyone knows how to do it, plz help.

Thanks in Advance.

Posted 20-Jul-11 1:30am

ankur.mu

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Comments

Herman<T>.Instance 20-Jul-11 7:34am

first you needed 8bpp to 16 and now 32 to 8.......??

ankur.mu 20-Jul-11 7:37am

actually when i read the bitmap image from an avi file using memory stream..the avi file is 8bpp but memory stream converts it into 32 bpp & adds random colors to the image. I want to retrive the 8bpp bitmap image in the same color & not with different random colors. Heres the code:

public Bitmap GetBitmap(int position){
if(position > countFrames){
throw new Exception("Invalid frame position: "+position);
}

Avi.AVISTREAMINFO streamInfo = GetStreamInfo(StreamPointer);

//Decompress the frame and return a pointer to the DIB
int dib = Avi.AVIStreamGetFrame(getFrameObject, firstFrame + position);
//Copy the bitmap header into a managed struct
Avi.BITMAPINFOHEADER bih = new Avi.BITMAPINFOHEADER();
bih = (Avi.BITMAPINFOHEADER)Marshal.PtrToStructure(new IntPtr(dib), bih.GetType());

if(bih.biSizeImage < 1){
throw new Exception("Exception in VideoStreamGetFrame");
}

//copy the image

byte[] bitmapData;
int address = dib + Marshal.SizeOf(bih);

Bitmap saveableBitmap;

if (bih.biBitCount < 16)
{
//copy palette and pixels
// Bitmap b = (Bitmap)Image.FromStream(aviStream);
bitmapData = new byte[bih.biSizeImage + Avi.PALETTE_SIZE];

}
else
{
//copy only pixels
bitmapData = new byte[bih.biSizeImage];
}

Marshal.Copy(new IntPtr(address), bitmapData, 0, bitmapData.Length);

//copy bitmap info
byte[] bitmapInfo = new byte[Marshal.SizeOf(bih)];
IntPtr ptr;
ptr = Marshal.AllocHGlobal(bitmapInfo.Length);
Marshal.StructureToPtr(bih, ptr, false);
address = ptr.ToInt32();
Marshal.Copy(new IntPtr(address), bitmapInfo, 0, bitmapInfo.Length);

Marshal.FreeHGlobal(ptr);

//create file header
Avi.BITMAPFILEHEADER bfh = new Avi.BITMAPFILEHEADER();
bfh.bfType = Avi.BMP_MAGIC_COOKIE;
bfh.bfSize = (Int32)(55 + bih.biSizeImage); //size of file as written to disk
bfh.bfReserved1 = 0;
bfh.bfReserved2 = 0;
bfh.bfOffBits = Marshal.SizeOf(bih) + Marshal.SizeOf(bfh);
if (bih.biBitCount < 16)
{
//There is a palette between header and pixel data
bfh.bfOffBits += Avi.PALETTE_SIZE;
}

//write a bitmap stream
BinaryWriter bw = new BinaryWriter(new MemoryStream());

//write header
bw.Write(bfh.bfType);
bw.Write(bfh.bfSize);
bw.Write(bfh.bfReserved1);
bw.Write(bfh.bfReserved2);
bw.Write(bfh.bfOffBits);
//write bitmap info
bw.Write(bitmapInfo);
//write bitmap data
bw.Write(bitmapData);

Bitmap bmp = (Bitmap)Image.FromStream(bw.BaseStream);
saveableBitmap = new Bitmap(bmp.Width, bmp.Height);
Graphics g = Graphics.FromImage(saveableBitmap);
g.DrawImage(bmp, 0, 0);
g.Dispose();
bmp.Dispose();

bw.Close();

return saveableBitmap;
}

Please help me I am new to C#.

Thanks in Advance.

ankur.mu 20-Jul-11 7:45am

ok! but when the compiler implicitly converts 8bpp to 32bpp when placing on clipboard or memory stream, why does it add random colors by itself?? Is there a way to avoid that?? THANKS!!

3 solutions

Solution 1

Look for the following search term on google:

convert 32 bpp bitmap into 8 bpp "C#"

Another job well done ;)

Posted 20-Jul-11 1:36am

Doc Lobster

Comments

ankur.mu 20-Jul-11 7:42am

ok! but when the compiler implicitly converts 8bpp to 32bpp when placing on clipboard or memory stream, why does it add random colors by itself?? Is there a way to avoid that??
THANKS!!

Solution 3

additional for error unsafe code:
Just go to project properties --> build--> check the checkbox "allow unsafe code".

Posted 14-Sep-14 22:59pm

Derz

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ankur.mu · Accepted Answer · 2011-07-20T21:34:00

Here is the code to this :

public static Bitmap ColorToGrayscale(Bitmap bmp)
{
int w = bmp.Width,
h = bmp.Height,
r, ic, oc, bmpStride, outputStride, bytesPerPixel;
PixelFormat pfIn = bmp.PixelFormat;
ColorPalette palette;
Bitmap output;
BitmapData bmpData, outputData;

//Create the new bitmap
output = new Bitmap(w, h, PixelFormat.Format8bppIndexed);

//Build a grayscale color Palette
palette = output.Palette;
for (int i = 0; i < 256; i++)
{
Color tmp = Color.FromArgb(255, i, i, i);
palette.Entries[ i ] = Color.FromArgb(255, i, i, i);
}
output.Palette = palette;

//No need to convert formats if already in 8 bit
if (pfIn == PixelFormat.Format8bppIndexed)
{
output = (Bitmap)bmp.Clone();

//Make sure the palette is a grayscale palette and not some other
//8-bit indexed palette
output.Palette = palette;

return output;
}

//Get the number of bytes per pixel
switch (pfIn)
{
case PixelFormat.Format24bppRgb: bytesPerPixel = 3; break;
case PixelFormat.Format32bppArgb: bytesPerPixel = 4; break;
case PixelFormat.Format32bppRgb: bytesPerPixel = 4; break;
default: throw new InvalidOperationException("Image format not supported");
}

//Lock the images
bmpData = bmp.LockBits(new Rectangle(0, 0, w, h), ImageLockMode.ReadOnly,
pfIn);
outputData = output.LockBits(new Rectangle(0, 0, w, h), ImageLockMode.WriteOnly,
PixelFormat.Format8bppIndexed);
bmpStride = bmpData.Stride;
outputStride = outputData.Stride;

//Traverse each pixel of the image
unsafe
{
byte* bmpPtr = (byte*)bmpData.Scan0.ToPointer(),
outputPtr = (byte*)outputData.Scan0.ToPointer();

if (bytesPerPixel == 3)
{
//Convert the pixel to it's luminance using the formula:
// L = .299*R + .587*G + .114*B
//Note that ic is the input column and oc is the output column
for (r = 0; r < h; r++)
for (ic = oc = 0; oc < w; ic += 3, ++oc)
outputPtr[r * outputStride + oc] = (byte)(int)
(0.299f * bmpPtr[r * bmpStride + ic] +
0.587f * bmpPtr[r * bmpStride + ic + 1] +
0.114f * bmpPtr[r * bmpStride + ic + 2]);
}
else //bytesPerPixel == 4
{
//Convert the pixel to it's luminance using the formula:
// L = alpha * (.299*R + .587*G + .114*B)
//Note that ic is the input column and oc is the output column
for (r = 0; r < h; r++)
for (ic = oc = 0; oc < w; ic += 4, ++oc)
outputPtr[r * outputStride + oc] = (byte)(int)
((bmpPtr[r * bmpStride + ic] / 255.0f) *
(0.299f * bmpPtr[r * bmpStride + ic + 1] +
0.587f * bmpPtr[r * bmpStride + ic + 2] +
0.114f * bmpPtr[r * bmpStride + ic + 3]));
}
}

//Unlock the images
bmp.UnlockBits(bmpData);
output.UnlockBits(outputData);

return output;
}