Develop Reference

Resize Bitmap without losing original ratio on C#

This code basically takes an image and crops it according to a detected image (in this case, it detects a decentralized fingerprint, and returns a new Bitmap with the fingerprint centered and cropped).
It turns out that, depending on the image, each resulting Bitmap will have a different size (for example, my test is returning a 425x448 Bitmap, since the identified image has that size), when in fact I need the image to return with a specific size (512x512).
I've already tried to change all the height and width variables of the code, but none satisfy this desired condition. Either it creates a Bitmap with the size of 512x512 and stretches the original image (violating the original ratio), or it creates a 512x512 Bitmap with the cropped image but with a black border on the right and bottom sides.
Any hints of what can be changed or included in the code?
Edit: More clearly, I need to create a 512x512 canvas for the 425x448 image without changing the size or dimensions of the image (it should be 425x448 inside a 512x512 canvas).
private byte[] GetAndCropImage(byte[] image, IEnumerable<YoloItem> yoloItems)
{
byte[] imageRet = null;
var topYoloItem = yoloItems?.Where(x => x.Confidence >= 0.30).OrderByDescending(x => x.Confidence).First();
MemoryStream ms = new MemoryStream(image);
Bitmap src = new Bitmap(ms);
Rectangle cropRect = new Rectangle(topYoloItem.X, topYoloItem.Y, topYoloItem.Width, topYoloItem.Height);
Bitmap target = new Bitmap(cropRect.Width, cropRect.Height);
using (Graphics g = Graphics.FromImage(target))
{
g.DrawImage(src, new Rectangle(0, 0, target.Width, target.Height),
cropRect,
GraphicsUnit.Pixel);
}
target.SetResolution(512, 512);
ImageConverter converter = new ImageConverter();
imageRet = (byte[])converter.ConvertTo(target, typeof(byte[]));
return imageRet;
}

Insufficient buffer size using WriteableBitmap?

I am modifying the ColorBasic Kinect example in order to display an image overlaid to the video stream. So what I've done is to load an image with transparent background (now a GIF but it may change), and write to the displayed bitmap.
The error I'm getting is that the buffer I'm writing to is too small.
I cannot see what the actual error is (I'm a complete newbie in XAML/C#/Kinect), but the WriteableBitmap is 1920x1080, and the bitmap I want to copy is 200x200, so why am I getting this error? I cannot see how a transparent background could be of any harm, but I am beginning to suspect that...
Note that without the last WritePixels, the code works and I see the webcam's output. My code follows.
The overlay image:
public BitmapImage overlay = new BitmapImage(new Uri("C:\\users\\user\\desktop\\something.gif"));
The callback function that displays the Kinect's webcam (see the default example ColorBasic) with my very small modifications:
private void Reader_ColorFrameArrived(object sender, ColorFrameArrivedEventArgs e)
{
// ColorFrame is IDisposable
using (ColorFrame colorFrame = e.FrameReference.AcquireFrame())
{
if (colorFrame != null)
{
FrameDescription colorFrameDescription = colorFrame.FrameDescription;
using (KinectBuffer colorBuffer = colorFrame.LockRawImageBuffer())
{
this.colorBitmap.Lock();
// verify data and write the new color frame data to the display bitmap
if ((colorFrameDescription.Width == this.colorBitmap.PixelWidth) && (colorFrameDescription.Height == this.colorBitmap.PixelHeight))
{
colorFrame.CopyConvertedFrameDataToIntPtr(
this.colorBitmap.BackBuffer,
(uint)(colorFrameDescription.Width * colorFrameDescription.Height * 4),
ColorImageFormat.Bgra);
this.colorBitmap.AddDirtyRect(new Int32Rect(0, 0, this.colorBitmap.PixelWidth, this.colorBitmap.PixelHeight));
}
if(this.overlay != null)
{
// Calculate stride of source
int stride = overlay.PixelWidth * (overlay.Format.BitsPerPixel / 8);
// Create data array to hold source pixel data
byte[] data = new byte[stride * overlay.PixelHeight];
// Copy source image pixels to the data array
overlay.CopyPixels(data, stride, 0);
this.colorBitmap.WritePixels(new Int32Rect(0, 0, overlay.PixelWidth, overlay.PixelHeight), data, stride, 0);
}
this.colorBitmap.Unlock();
}
}
}
}

Your overlay.Format.BitsPerPixel / 8 will be 1 (because it's a gif), but you're trying to copy it to something that is not a gif, probably BGRA (32 bit). Thus you got a huge difference in size (4x).
.WritePixels should take in the stride value of the destination buffer, but you past it the stride value of the overlay (this can cause weird problems as well).
And finally, even if it went 100% smooth your overlay will not actually "overlay" anything, it will replace -- since I don't see any alpha bending math in your code.
Switch your .gif to a .png (32bit) and see if that helps.
Also, if you're looking for an AlphaBltMerge type code: I wrote the entire thing here.. it's very easy to understand.
Merge 2 - 32bit Images with Alpha Channels

C# Capture screen to 8-bit (256 color) bitmap

I'm using this code to capture the screen:
public Bitmap CaptureWindow(IntPtr handle)
{
// get te hDC of the target window
IntPtr hdcSrc = User32.GetWindowDC(handle);
// get the size
User32.RECT windowRect = new User32.RECT();
User32.GetWindowRect(handle, ref windowRect);
int width = windowRect.right - windowRect.left;
int height = windowRect.bottom - windowRect.top;
// create a device context we can copy to
IntPtr hdcDest = GDI32.CreateCompatibleDC(hdcSrc);
// create a bitmap we can copy it to,
// using GetDeviceCaps to get the width/height
IntPtr hBitmap = GDI32.CreateCompatibleBitmap(hdcSrc, width, height);
// select the bitmap object
IntPtr hOld = GDI32.SelectObject(hdcDest, hBitmap);
// bitblt over
GDI32.BitBlt(hdcDest, 0, 0, width, height, hdcSrc, 0, 0, GDI32.SRCCOPY);
// restore selection
GDI32.SelectObject(hdcDest, hOld);
// clean up
GDI32.DeleteDC(hdcDest);
User32.ReleaseDC(handle, hdcSrc);
// get a .NET image object for it
Bitmap img = Image.FromHbitmap(hBitmap);
// free up the Bitmap object
GDI32.DeleteObject(hBitmap);
return img;
}
I then want to convert the bitmap to 256 colors (8 bit). I tried this code but get an error about not being able to create an Image from an indexed bitmap format:
Bitmap img8bit = new Bitmap(img.Width,img.Height,
System.Drawing.Imaging.PixelFormat.Format8bppIndexed);
Graphics g = Graphics.FromImage(img8bit);
g.DrawImage(img,new Point(0,0));
I did see some examples to convert bitmaps between different formats, but in my case I'm looking for the best way to do this while capturing from the screen. For example, if there is a method that will work better by creating an 8-bit bitmap to begin with and then blit the screen to that, that would be preferred over caputring screen to comptible bitmap first and then converting it. Unless it's better to capture then convert anyway.
I have a program written in C++ using Borland Builder 6.0 VCL, and I'm trying to memic that. In that case it is a simple matter of setting the pixel format for VCL's TBitmap object. I notice Bitmap.PixelFormat is read-only in .NET, ugh.
Update: In my case I don't think the answer is as complex as some other usage that requires figuring out the best palette entries, because Graphics.GetHalftonePalette using the screen DC should be fine, since my original bitmap comes from the screen, not just any random bitmap that might come from a file/email/download/etc. I beleive there is something that can be done with maybe 20 lines of code that involves DIBs and GetHalftonePalette -- just can't find it yet.

Converting a full color bitmap to 8bpp is a difficult operation. It requires creating a histogram of all the colors in the image and creating a palette that contains an optimized set of colors that best map to the original colors. Then using a technique like dithering or error diffusion to replace the pixels whose colors don't have an exact match with the palette.
This is best left to a professional graphics library, something like ImageTools. There is one cheap way that can be tricked in the .NET framework. You can use the GIF encoder, a file format that has 256 colors. The result isn't the greatest, it uses dithering and that can be pretty visible sometimes. Then again, if you really cared about image quality then you wouldn't use 8bpp anyway.
public static Bitmap ConvertTo8bpp(Image img) {
var ms = new System.IO.MemoryStream(); // Don't use using!!!
img.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
ms.Position = 0;
return new Bitmap(ms);
}

Capture the screen using a regular PixelFormat and then use Bitmap.Clone() to convert it to an optimized 256 indexed color like this:
public static Bitmap CaptureScreen256()
{
Rectangle bounds = SystemInformation.VirtualScreen;
using (Bitmap Temp = new Bitmap(bounds.Width, bounds.Height, PixelFormat.Format24bppRgb))
{
using (Graphics g = Graphics.FromImage(Temp))
{
g.CopyFromScreen(0, 0, 0, 0, Temp.Size);
}
return Temp.Clone(new Rectangle(0, 0, bounds.Width, bounds.Height), PixelFormat.Format8bppIndexed);
}
}

How to get a part of an image and use it as a separate image?

I have a tileset but all the tiles are in one image. I want to get every tile in some kind of bitmap or image array.
Is there some kind of method that does that?

Use Bitmap.Clone(Rectangle, PixelFormat) , Here we can set PixelFormat of new image and the size of the rectangle
// Create a Bitmap object from a file.
Bitmap myBitmap = new Bitmap("Grapes.jpg");
// Clone a portion of the Bitmap object.
Rectangle cloneRect = new Rectangle(0, 0, 100, 100);
Bitmap cloneBitmap = myBitmap.Clone(cloneRect, myBitmap.PixelFormat);
// Draw the cloned portion of the Bitmap object.
e.Graphics.DrawImage(cloneBitmap, 0, 0);

Create Bitmap from a byte array of pixel data

This question is about how to read/write, allocate and manage the pixel data of a Bitmap.
Here is an example of how to allocate a byte array (managed memory) for pixel data and creating a Bitmap using it:
Size size = new Size(800, 600);
PixelFormat pxFormat = PixelFormat.Format8bppIndexed;
//Get the stride, in this case it will have the same length of the width.
//Because the image Pixel format is 1 Byte/pixel.
//Usually stride = "ByterPerPixel"*Width
//But it is not always true. More info at bobpowell.
int stride = GetStride(size.Width, pxFormat);
byte[] data = new byte[stride * size.Height];
GCHandle handle = GCHandle.Alloc(data, GCHandleType.Pinned);
Bitmap bmp = new Bitmap(size.Width, size.Height, stride,
pxFormat, handle.AddrOfPinnedObject());
//After doing your stuff, free the Bitmap and unpin the array.
bmp.Dispose();
handle.Free();
public static int GetStride(int width, PixelFormat pxFormat)
{
//float bitsPerPixel = System.Drawing.Image.GetPixelFormatSize(format);
int bitsPerPixel = ((int)pxFormat >> 8) & 0xFF;
//Number of bits used to store the image data per line (only the valid data)
int validBitsPerLine = width * bitsPerPixel;
//4 bytes for every int32 (32 bits)
int stride = ((validBitsPerLine + 31) / 32) * 4;
return stride;
}
I thought that the Bitmap would make a copy of the array data, but it actually points to the same data. Was you can see:
Color c;
c = bmp.GetPixel(0, 0);
Console.WriteLine("Color before: " + c.ToString());
//Prints: Color before: Color [A=255, R=0, G=0, B=0]
data[0] = 255;
c = bmp.GetPixel(0, 0);
Console.WriteLine("Color after: " + c.ToString());
//Prints: Color after: Color [A=255, R=255, G=255, B=255]
Questions:
Is it safe to do create a bitmap from a byte[] array (managed memory) and free() the GCHandle? If it is not safe, Ill need to keep a pinned array, how bad is that to GC/Performance?
Is it safe to change the data (ex: data[0] = 255;)?
The address of a Scan0 can be changed by the GC? I mean, I get the Scan0 from a locked bitmap, then unlock it and after some time lock it again, the Scan0 can be different?
What is the purpose of ImageLockMode.UserInputBuffer in the LockBits method? It is very hard to find info about that! MSDN do not explain it clearly!
EDIT 1: Some followup
You need to keep it pinned. Will it slow down the GC? I've asked it here. It depends on the number of images and its sizes. Nobody have gave me a quantitative answer. It seams that it is hard to determine.
You can also alloc the memory using Marshal or use the unmanaged memory allocated by the Bitmap.
I've done a lot of test using two threads. As long as the Bitmap is locked it is ok. If the Bitmap is unlock, than it is not safe! My related post about read/write directly to Scan0. Boing's answer "I already explained above why you are lucky to be able to use scan0 outside the lock. Because you use the original bmp PixelFormat and that GDI is optimized in that case to give you the pointer and not a copy. This pointer is valid until the OS will decide to free it. The only time there is a guarantee is between LockBits and UnLockBits. Period."
Yeah, it can happen, but large memory regions are treated different by the GC, it moves/frees this large object less frequently. So it can take a while to GC move this array. From MSDN: "Any allocation greater than or equal to 85,000 bytes goes on the large object heap (LOH)" ... "LOH is only collected during a generation 2 collection". .NET 4.5 have Improvements in LOH.
This question have been answered by #Boing. But I'm going to admit. I did not fully understand it. So if Boing or someone else could please clarify it, I would be glad. By the way, Why I can't just directly read/write to Sca0 without locking? => You should not write directly to Scan0 because Scan0 points to a copy of the Bitmap data made by the unmanaged memory (inside GDI). After unlock, this memory can be reallocate to other stuff, its not certain anymore that Scan0 will point to the actual Bitmap data. This can be reproduced getting the Scan0 in a lock, unlock, and do some rotate-flit in the unlocked bitmap. After some time, Scan0 will point to an invalid region and you will get an exception when trying to read/write to its memory location.

Its safe if you marshal.copy data rather than setting scan0 (directly or via that overload of BitMap()). You don't want to keep managed objects pinned, this will constrain the garbage collector.
If you copy, perfectly safe.
The input array is managed and can be moved by the GC, scan0 is an unmanaged pointer that would get out of date if the array moved. The Bitmap object itself is managed but sets the scan0 pointer in Windows via a handle.
ImageLockMode.UserInputBuffer is? Apparently it can be passed to LockBits, maybe it tells Bitmap() to copy the input array data.
Example code to create a greyscale bitmap from array:
var b = new Bitmap(Width, Height, PixelFormat.Format8bppIndexed);
ColorPalette ncp = b.Palette;
for (int i = 0; i < 256; i++)
ncp.Entries[i] = Color.FromArgb(255, i, i, i);
b.Palette = ncp;
var BoundsRect = new Rectangle(0, 0, Width, Height);
BitmapData bmpData = b.LockBits(BoundsRect,
ImageLockMode.WriteOnly,
b.PixelFormat);
IntPtr ptr = bmpData.Scan0;
int bytes = bmpData.Stride*b.Height;
var rgbValues = new byte[bytes];
// fill in rgbValues, e.g. with a for loop over an input array
Marshal.Copy(rgbValues, 0, ptr, bytes);
b.UnlockBits(bmpData);
return b;

Concerning your question 4: The ImageLockMode.UserInputBuffer can give you the control of the allocating process of those huge amount of memory that could be referenced into a BitmapData object.
If you choose to create yourself the BitmapData object you can avoid a Marshall.Copy. You will then have to use this flag in combinaison with another ImageLockMode.
Beware that it is a complicated business, specially concerning Stride
and PixelFormat.
Here is an example that would get in one shot the content of 24bbp buffer onto a BitMap and then in one another shot read it back into another buffer into 48bbp.
Size size = Image.Size;
Bitmap bitmap = Image;
// myPrewrittenBuff is allocated just like myReadingBuffer below (skipped for space sake)
// But with two differences: the buff would be byte [] (not ushort[]) and the Stride == 3 * size.Width (not 6 * ...) because we build a 24bpp not 48bpp
BitmapData writerBuff= bm.LockBits(new Rectangle(0, 0, size.Width, size.Height), ImageLockMode.UserInputBuffer | ImageLockMode.WriteOnly, PixelFormat.Format24bppRgb, myPrewrittenBuff);
// note here writerBuff and myPrewrittenBuff are the same reference
bitmap.UnlockBits(writerBuff);
// done. bitmap updated , no marshal needed to copy myPrewrittenBuff
// Now lets read back the bitmap into another format...
BitmapData myReadingBuffer = new BitmapData();
ushort[] buff = new ushort[(3 * size.Width) * size.Height]; // ;Marshal.AllocHGlobal() if you want
GCHandle handle= GCHandle.Alloc(buff, GCHandleType.Pinned);
myReadingBuffer.Scan0 = Marshal.UnsafeAddrOfPinnedArrayElement(buff, 0);
myReadingBuffer.Height = size.Height;
myReadingBuffer.Width = size.Width;
myReadingBuffer.PixelFormat = PixelFormat.Format48bppRgb;
myReadingBuffer.Stride = 6 * size.Width;
// now read into that buff
BitmapData result = bitmap.LockBits(new Rectangle(0, 0, size.Width, size.Height), ImageLockMode.UserInputBuffer | ImageLockMode.ReadOnly, PixelFormat.Format48bppRgb, myReadingBuffer);
if (object.ReferenceEquals(result, myReadingBuffer)) {
// Note: we pass here
// and buff is filled
}
bitmap.UnlockBits(result);
handle.Free();
// use buff at will...
If you use ILSpy you'll see that this method link to GDI+ and those methods helps are more complete.
You may increase performance by using your own memory scheme, but
beware that Stride may need to have some alignment to get the best
performance.
You then will be able to go wild for example allocating huge virtual memory mapped scan0 and blit them quite efficiently.
Note that pinning huge array (and especially a few) won't be a burden to the GC and will allow you to manipulate the byte/short in a totally safe way (or unsafe if you seek speed)

I'm not sure if there is a reason you're doing it the way you are. Maybe there is. It seems like you're off the beaten path enough so that you might be trying to do something more advanced than what the title of your question implies...
However, the traditional way of creating a Bitmap from a Byte array is:
using (MemoryStream stream = new MemoryStream(byteArray))
{
Bitmap bmp = new Bitmap(stream);
// use bmp here....
}

Here is a sample code i wrote to convert byte array of pixels to an 8 bits grey scale image(bmp)
this method accepts the pixel array, image width, and height as arguments
//
public Bitmap Convert2Bitmap(byte[] DATA, int width, int height)
{
Bitmap Bm = new Bitmap(width,height,PixelFormat.Format24bppRgb);
var b = new Bitmap(width, height, PixelFormat.Format8bppIndexed);
ColorPalette ncp = b.Palette;
for (int i = 0; i < 256; i++)
ncp.Entries[i] = Color.FromArgb(255, i, i, i);
b.Palette = ncp;
for (int y = 0; y < height; y++)
{
for (int x = 0; x < width; x++)
{
int Value = DATA[x + (y * width)];
Color C = ncp.Entries[Value];
Bm.SetPixel(x,y,C);
}
}
return Bm;
}