How do I save an UIImage as BMP?

Question

Can I save (write to file) UIImage object as bitmap file (.bmp extension) in iPhone’s document directory?

Thanks in advance.

Answer 1

I don’t think BMP is supported on iPhone. Maybe somebody wrote a category for UIImage that does saving into BMP, but I don’t know about any. I guess You’ll have to get the bitmap data from the UIImage and write them yourself, BMP is quite a simple file format. All you have to do is write out the header and then the uncompressed data. The header is a structure called BITMAPINFOHEADER, see MSDN. Getting the bitmap data of an UIImage is described in Apple’s Technical Q&A1509.

Answer 2

Since BMP is not a compressed format, is this a good idea?

Presumably, image size is even more important on portable devices.

Answer 3

Right now I am not concerned about the size. Just want to know can I write image data as .bmp file.

Answer 4

Realize this is an old post, but in case someone finds it like I did looking for a solution. I basically needed to FTP UIImage as a small BMP, so I hacked this crude class in MonoTouch. I borrowed from zxing.Bitmap.cs and Example 1 from wikipedia BMP article. It appears to work. Might have been slicker to do an extension like AsBMP() or something like that. (I don't know what the objective-c equivalent is, but hopefully this is helpful to someone.)

using System;
using System.Drawing;
using System.Runtime.InteropServices;
using MonoTouch.Foundation;
using MonoTouch.UIKit;
using MonoTouch.CoreGraphics;
public class BitmapFileRGBA8888
{
    public byte[] Data; // data needs to be BGRA
    public const int PixelDataOffset = 54;

    public BitmapFileRGBA8888(UIImage image)
    {
        CGImage imageRef = image.CGImage;
        int width = imageRef.Width;
        int height = imageRef.Height;
        Initialize((uint)width, (uint)height);
        CGColorSpace colorSpace = CGColorSpace.CreateDeviceRGB();

        IntPtr rawData = Marshal.AllocHGlobal(height*width*4);
        CGContext context = new CGBitmapContext(
            rawData, width, height, 8, 4*width, colorSpace, CGImageAlphaInfo.PremultipliedLast
        );
        context.DrawImage(new RectangleF(0.0f,0.0f,(float)width,(float)height),imageRef); // RGBA

        byte[] pixelData = new byte[height*width*4];
        Marshal.Copy(rawData,pixelData,0,pixelData.Length);
        Marshal.FreeHGlobal(rawData);

        int di = PixelDataOffset;
        int si;
        for (int y = 0; y < height; y++)
        {
            si = (height-y-1) * 4 * width;
            for (int x = 0; x < width; x++)
            {
                CopyFlipPixel(pixelData, si, Data, di);
                di += 4; // destination marchs forward
                si += 4;
            }
        }
    }

    private void CopyFlipPixel(byte[] Src, int Src_offset, byte[] Dst, int Dst_offset)
    {
        int S = Src_offset;
        int D = Dst_offset + 2;
        Dst[D--] = Src[S++]; // R
        Dst[D--] = Src[S++]; // G
        Dst[D--] = Src[S++]; // B
        Dst[Dst_offset+3] = Src[S]; // alpha
    }

    private void Initialize(uint W, uint H)
    {
        uint RawPixelDataSize = W * H * 4;
        uint Size = RawPixelDataSize + 14 + 40;
        Data = new byte[Size];
        Data[0] = 0x42; Data[1] = 0x4D; // BITMAPFILEHEADER "BM"
        SetLong(0x2, Size); // file size
        SetLong(0xA, PixelDataOffset); // offset to pixel data
        SetLong(0xE, 40); // bytes in DIB header (BITMAPINFOHEADER)
        SetLong(0x12, W);
        SetLong(0x16, H);
        SetShort(0x1A, 1); // 1 plane
        SetShort(0x1C, 32); // 32 bits
        SetLong(0x22, RawPixelDataSize);
        SetLong(0x26, 2835); // h/v pixels per meter device resolution
        SetLong(0x2A, 2835);
    }

    private void SetShort(int Offset, UInt16 V)
    {
        var byts = BitConverter.GetBytes(V);
        if (!BitConverter.IsLittleEndian) Array.Reverse(byts);
        Array.Copy(byts,0,Data,Offset,byts.Length);
    }
    private void SetLong(int Offset, UInt32 V)
    {
        var byts = BitConverter.GetBytes(V);
        if (!BitConverter.IsLittleEndian) Array.Reverse(byts);
        Array.Copy(byts,0,Data,Offset,byts.Length);
    }
} // END CLASS

Basically

var Bmp = new BitmapFileRGBA8888(TempImage);
FTP.UploadBin(Bmp.Data, "test.bmp");        // or just write as binary file