Develop Reference

How to set pixels from original image to a new image?

usage
public UserControlTester()
{
InitializeComponent();
GetSetPixelsRandom();
}
code
private void GetSetPixelsRandom()
{
Bitmap img = new Bitmap(#"d:\\downloaded images\\test.gif");
for (int i = 0; i < img.Width; i++)
{
for (int j = 0; j < img.Height; j++)
{
Color pixel = img.GetPixel(i, j);
pixels.Add(pixel);
}
}
Bitmap img1 = new Bitmap(512,512);
for (int i = 0; i < img1.Width; i++)
{
for (int j = 0; j < img1.Height; j++)
{
for (int c = 0; c < pixels.Count; c++)
{
img1.SetPixel(i, j, pixels[c]);
}
}
}
img1.Save(#"d:\\downloaded images\\test3.gif");
}
Not sure if this is the right way. the inner loop with the variable c take a lot of time because there are more then 290,000 pixels.

Assuming you want to resize the image, this can be done easily. The Bitmap constructor has rescaling build in:
Bitmap img = new Bitmap(#"d:\\downloaded images\\test.gif");
Bitmap img1 = new Bitmap(img, new Size(512, 512));
img1.Save(#"d:\\downloaded images\\test3.gif");
If you do not want to resize but indeed take random pixels take a look at what at MickyD mentioned.
Note that in your provided example every pixel in the new image is effectively set to the last pixel in the pixels list because you loop over every pixel in pixels for every pixel in the new image.

Why AccessViolationException occurs when accessing pixels in WriteableBitmap Image?

I have a video stream from a camera to an Image in a WPF. I am trying to access the WritableBitMap Image pixel by pixel before displaying it. As a test I am trying to set the whole image to white or black. In both cases however, I get the AccessViolationException error.
I checked other posts and it seems that this error is very wide and not specific to my case. I can't seem to know why I am not getting this working.
So what is the best way to play with the pixels in my case? or why this is not working? Any help is appreciated
private async void UpdateMasterCameraPreview(IdsFrame frame)
{
if (masterImage != null)
frame.Image.CopyTo(masterImage);
else
masterImage = frame.Image.ToWriteableBitmap();
//BitmapImage temp = ConvertWriteableBitmapToBitmapImage(masterImage);
WriteableBitmap temp = masterImage;
// Here I get the exception, on every pixel access
for (int y = 0; y < temp.PixelHeight; y++)
for (int x = 0; x < temp.PixelWidth; x++)
temp.SetPixel(x, y, 255);
masterImage = temp;
masterImage.Lock();
masterImage.AddDirtyRect(new Int32Rect(0, 0, masterImage.PixelWidth, masterImage.PixelHeight));
masterImage.Unlock();
if (OnMasterFrameCaptured != null)
OnMasterFrameCaptured(this, new CameraFrameCapturedArgs(CameraType.Master, masterImage));
}

You have swapped X and Y, i represents height, j represents width, then you shouldcall SetPixel like:
temp.SetPixel(j, i, 255);
On cases like this is better to use meaningful names for variables, like X and Y.

I ended up using the answer of this post. I now can edit raw pixel data of any WriteableBitmap image before sending it to image control in WPF. Below is what I exactly used but here I just convert every frame to some transparency under a condition:
public void ConvertImage(ref WriteableBitmap Wbmp)
{
int width = Wbmp.PixelWidth;
int height = Wbmp.PixelHeight;
int stride = Wbmp.BackBufferStride;
int bytesPerPixel = (Wbmp.Format.BitsPerPixel + 7) / 8;
unsafe
{
byte* pImgData = (byte*)Wbmp.BackBuffer;
// set alpha to transparent for any pixel with red < 0x88 and invert others
int cRowStart = 0;
int cColStart = 0;
for (int row = 0; row < height; row++)
{
cColStart = cRowStart;
for (int col = 0; col < width; col++)
{
byte* bPixel = pImgData + cColStart;
UInt32* iPixel = (UInt32*)bPixel;
if (bPixel[2 /* bgRa */] < 0x44)
{
// set to 50% transparent
bPixel[3 /* bgrA */] = 0x7f;
}
else
{
// invert but maintain alpha
*iPixel = *iPixel ^ 0x00ffffff;
}
cColStart += bytesPerPixel;
}
cRowStart += stride;
}
}
}
And the routine of using it is like this:
masterImage.Lock();
ConvertImage(ref masterImage);
masterImage.AddDirtyRect(new Int32Rect(0, 0, masterImage.PixelWidth, masterImage.PixelHeight));
masterImage.Unlock();

Unable to read pixels from bitmap using Xamarin

I thought trying to get pixel data would be a simple thing to do, but it has become a big headache.
My original goal is to go through every pixel in the bitmap and then do some modification with the data. However every time I try to get the pixel I get 0 as a result. It feels like the image is not going to the bitmap. Am I missing something in my code or the code in general wrong?
protected override void OnCreate(Bundle bundle)
{
base.OnCreate(bundle);
SetContentView(Resource.Layout.Main);
var imageView1 = FindViewById<ImageView>(Resource.Id.imageView1);
imageView1.SetImageResource(Resource.Drawable.pic1);
Bitmap b = BitmapFactory.DecodeResource(Resources,Resource.Drawable.pic1);
b = Bitmap.CreateBitmap(b);
imageView1.SetImageBitmap(b);
for (int i = 0; i < width; i++)
{
for (int j = 0; j < height; j++)
{
int pixel = b.GetPixel(i, j);
int A = Color.GetAlphaComponent(pixel);
int R = Color.GetRedComponent(pixel);
int G = Color.GetGreenComponent(pixel);
int B = Color.GetBlueComponent(pixel);
}
}
}

Needed to use
Bitmap b = BitmapFactory.DecodeResource(Resources,Resource.Drawable.pic1);

for (int i = 0; i < width; i++)
{
for (int j = 0; j < height; j++)
{
Color pixel = new Color(b.GetPixel(i, j));
int A = Color.GetAlphaComponent(pixel);
int R = Color.GetRedComponent(pixel);
int G = Color.GetGreenComponent(pixel);
int B = Color.GetBlueComponent(pixel);
}
}

MSE calculation for grayscale images

I have two images(original and noisy). I'm calculating PSNR. I kinda did it for color RGB images, but i don't know how to do it with grayscale. As i read, MSE calculation is different. For RGB i'm doing it like you can see in following code (I'm using Visual C#):
for (int i = 0; i < bmp1.Width; i++)
{
for (int j = 0; j < bmp1.Height; j++)
{
mseR += Math.Pow(bmp1.GetPixel(i, j).R - bmp2.GetPixel(i, j).R, 2);
mseG += Math.Pow(bmp1.GetPixel(i, j).G - bmp2.GetPixel(i, j).G, 2);
mseB += Math.Pow(bmp1.GetPixel(i, j).B - bmp2.GetPixel(i, j).B, 2);
}
}
mse = (mseR + mseG + mseB) / ((bmp1.Width * bmp1.Height) * 3);
Here I am manipulating with R,G,B of pixels.But i don't know what should i take in case of grayscale images. Can I use RGB aswell, because it actually gives some results, or i should take something else?

To make grayscale you can make the picture out of averages (no need to vary your implementation). I'm assuming your images are bmp1 = grayImage and bmp2 = noisy image.
for (int i = 0; i < bmp1.Width; i++)
{
for (int j = 0; j < bmp1.Height; j++)
{
// As a grayscale image has rthe same color on all RGB just pick one
int gray1 = bmp1.GetPixel(i, j).R;
int gray2 = bmp2.GetPixel(i, j).R;
double sum = Math.Pow(gray1 - gray2, 2)
mseGray += sum;
}
}
mse = (mseGray) / ((bmp1.Width * bmp1.Height) * 3);
Also getting pixels one at a time is a slow process look into using the indexes, and a optimization in the loop. It should give about a tenfold in performance.
You need to make the bitmap into an indexable img, I'm assuming its BitmapSource for this example. the interesting part is the loop and the index building and not the precode, the precode is just to make the image indexable.
var height = bmp1.Height;
var width = bmp1.Width;
var pixelBytes1 = new byte[height * width * 4];
var pixelBytes2 = new byte[height * width * 4];
bmp1.CopyPixels(pixelBytes1, stride, 0);
bmp2.CopyPixels(pixelBytes2, stride, 0);
for (int x = 0; x < width; x++)
{
int woff = x * height;
for (int y = 0; y < height; y++)
{(R*0.3 + G*0.59+ B*0.11)
int index = woff + y;
int gray1 = bmp1[index];
int gray2 = bmp2[index];
double sum = Math.Pow(gray1 - gray2, 2)
mseGray += sum;
}
}
mse = (mseGray) / ((bmp1.Width * bmp1.Height) * 3);
EDIT:
http://www.mathworks.com/matlabcentral/answers/49906-how-to-calculate-psnr-of-compressed-images-and-how-to-compare-psnr-of-images-compressed-by-two-diff
I'm having an issue with your implementation of PSNR though im thinking its not per definition
here is an example from java (very similar to C#)
http://www.cyut.edu.tw/~yltang/program/Psnr.java

Work with matrix in emgu cv

I have a matrix (2D) of an image in EMGU cv,how can I fill the rest of the matrix with zeros but keep a certain area(rectangle) with the original data?

Method 1
One way to achieve what your after is to access the Data of the Matrix directly and set the values to '0' the following code will set the lower quarter of 'My_Matrix' to 0 where all other values will remain 20.
Matrix<double> My_Matrix_Image = new Matrix<double>(8,10);
My_Matrix_Image.SetValue(20); //set all values of Matrix to 20
for(int i = 4; i< My_Matrix_Image.Height; i++)
{
for (int j = 5; j < My_Matrix_Image.Width; j++)
{
My_Matrix_Image.Data[i,j] = 0;
}
}
To achieve what you wanted from your comment you must still take the same approach My_Matrix_Image would contain your image data values from 0-255 I will give solutions for both greyscale and colour images. I will keep the first 30 rows of the image and set the rest to zero the image will be 100x100 pixels in size. Change j = 30 to alter the amount of rows.
First Colour Image Matrix there will be 3 channels Red, Green and Blue:
for(int i = 0; i< My_Matrix_Image.Height; i++)
{
for (int j = 30; j < My_Matrix_Image.Width; j++)
{
My_Matrix_Image.Data[i,j,0] = 0; //RED
My_Matrix_Image.Data[i,j,1] = 0; //GREEN
My_Matrix_Image.Data[i,j,2] = 0; //BLUE
}
}
Greyscale Image Matrices are easier as there will be 1 channel:
for(int i = 0; i< My_Matrix_Image.Height; i++)
{
for (int j = 30; j < My_Matrix_Image.Width; j++)
{
My_Matrix_Image.Data[i,j,0] = 0;
}
}
Now lets assume you want to keep the Middle 40 Rows you will have to add an additional loop remember this is the only way with a Matrix I have only provided the colour image example as you can see it starts getting a little messy and Method 2 may be better:
for(int i = 0; i< My_Matrix_Image.Height; i++)
{
//LOOP 1 SET THE FRONT ROWS
for (int j = 0; j<40; j++)
{
My_Matrix_Image.Data[i,j,0] = 0; //RED
My_Matrix_Image.Data[i,j,1] = 0; //GREEN
My_Matrix_Image.Data[i,j,2] = 0; //BLUE
}
// LOOP 2 SET THE BACK ROWS
for (int j = 60; j < My_Matrix_Image.Width; j++)
{
My_Matrix_Image.Data[i,j,0] = 0; //RED
My_Matrix_Image.Data[i,j,1] = 0; //GREEN
My_Matrix_Image.Data[i,j,2] = 0; //BLUE
}
}
Method 2
Now lets assume you do want to keep a rectangle of data. Creating 6 Loops is complex and inefficient so here is what you could do.
//Make a Blank Copy of your Image this will be automatically full of zeros
Matrix<double> My_Image_Copy = My_Image_Matrix.CopyBlank();
//Now copy the data you want to keep from your original image into you blank copy
for(int i = 40; i< 60; i++)
{
for (int j = 40; j < 60; j++)
{
My_Image_Copy.Data[i,j,0] = My_Matrix_Image.Data[i,j,0]; //RED
My_Image_Copy.Data[i,j,1] = My_Matrix_Image.Data[i,j,1]; //GREEN
My_Image_Copy.Data[i,j,2] = My_Matrix_Image.Data[i,j,2]; //BLUE
}
}
The above code will copy the centre 20x20 pixels from an image you can obviously change this to copy whole rows by using for(int i = 0; i< My_Matrix_Image.Height; i++)
Much better I'm sure you will agree.
Alternative
Now while you are using a Matrix to store you data using a Image construct makes coding a little simpler. While this may not be relevant to you it may be to others.
If you use Image or alternative to store your Image data then this can be achieved by:
Image<Gray, Byte> My_Image = new Image<Gray, byte>(openfile.FileName);
Image<Gray, Byte> My_Image_Copy = My_Image.CopyBlank();
Rectangle Store_ROI = my_image.ROI; //Only need one as both images same size
My_Image.ROI = new Rectangle(50, 50, 100, 100);
My_Image_Copy.ROI = new Rectangle(50, 50, 100, 100);
My_Image_Copy = My_Image.Copy(); //This will only copy the Region Of Interest
//Reset the Regions Of Interest so you will now operate on the whole image
My_Image.ROI = Store_ROI;
My_Image_Copy.ROI = Store_ROI;
Now this the same as Method 2 but you don't need to sort write out loops where errors can occur.
Hope this correction answers your question,
Cheers
Chris

// Rectangle's parameters
static int x = 3;
static int y = 3;
static int width = 2;
static int height = 2;
Rectangle specificArea = new Rectangle(x, y, width, height);
// Your image matrix; 20x20 just a sample
Matrix<int> imageMatrix = new Matrix<int>(20, 20);
public Matrix<int> cropMatrix()
{
// Crop a specific area from image matrix
Matrix<int> specificMatrix = imageMatrix.GetSubRect(specificArea);
// Matrix with full of zeros and same size with imageMatrix
Matrix<int> croppedMatrix = imageMatrix.CopyBlank();
for (int i = x; i < x+width; i++)
{
for (int j = y; j < y+height; j++)
{
// Set croppedMatrix with saved values
croppedMatrix[i, j] = specificMatrix[i-x, j-y];
}
}
return croppedMatrix;
}