Two reason to scale grayscale image without interpolation:
- intepolation add new color in image
- .net GDI mechanism can’t return image in 8 bits – pixel format
When a image is scaled with a scaling factor greater then 1, the image get black pixels,
to fill black pixels exist 3 well known algorithms
- Nearest neighbor
- Bilinear
- Bicubic
all 3 algorithms add new color in image
Idea of scalig iamge without interpolation is:
- to create an temprorary image with dimension computed with scaling factor
- in temporary image will be a group of pixels for evry pixel from original image
- to scale temporary image to target image that is small then target image; will get no black pixel when scaling with fator small than 1
With .net Bitmap object the SetPixel method does not work in image with indexed pixel format, shall to work with bitmap data.
//get original bmp data array Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height); System.Drawing.Imaging.BitmapData bmpData = bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite, bmp.PixelFormat); // Get the address of the first line. IntPtr ptr = bmpData.Scan0; // Declare an array to hold the bytes of the bitmap. int bytes = bmpData.Stride * bmp.Height; byte[] pixelValues = new byte[bytes]; // Copy the pixel values into the array. System.Runtime.InteropServices.Marshal.Copy(ptr, pixelValues, 0, bytes); //Unlock the bits. bmp.UnlockBits(bmpData); |
The scaling factor for temporary image will be the smallest integer greater then target scaling factor
//compute integer scaling factor int integerScalingFactor = ((int)scalingFactor); if (integerScalingFactor < scalingFactor) { integerScalingFactor++; } |
Create temporary image matrix
Temporary image never be a image, only a byte matrix that hold values of pixels
//foreach pixel from original image will be put integerScalingFactor pixels in temporary image int indexPixelValue = 0; for (int indexYOriginal = 0, indexY = 0; indexYOriginal < yOriginal; indexYOriginal++,indexY += integerScalingFactor) { int indexXOriginal = 0; for (int indexX = 0; indexXOriginal < xOriginal; indexXOriginal++, indexX += integerScalingFactor) { for (int indexYFactor = 0; indexYFactor < integerScalingFactor; indexYFactor++) { for (int indexXFactor = 0; indexXFactor < integerScalingFactor; indexXFactor++) { integerScaledMatrix[(indexY + indexYFactor), (indexX + indexXFactor)] = pixelValues[indexPixelValue]; } } indexPixelValue++; } indexPixelValue += bmpDataStride - indexXOriginal; } |
Compute scaling factor for temporary image to target image
Get scaling factor from division operation between target dimension and actual dimension
//scale integerScaledMatrix to target iamge dimension float scalingFactorFinalMatrix = ((float)scaledHeight) / ((float)heightScaledMatrix); |
Fill target image pixel data scaling teporary image.
For evry pixel from temporary image get new index in target image using scaling factor
//pixelValuesScaled is target image for (int y = 0; y < heightScaledMatrix; y++) { for (int x = 0; x < widthScaledMatrix; x++) { //with Convert.ToInt32 get the nearest index by floating index int yScalata = Convert.ToInt32(y * scalingFactorFinalMatrix); int xScalata = Convert.ToInt32(x * scalingFactorFinalMatrix); if (yScalata < bmpScaled.Height && xScalata < bmpScaled.Width) { pixelValuesScaled[bmpDataScaled.Stride * yScalata + xScalata] = integerScaledMatrix[y, x]; } } } |
At the end set target image data and the color palette
//Copy array data of scaled image in bmpData System.Runtime.InteropServices.Marshal.Copy(pixelValuesScaled, 0, ptrScaled, bytesScaled); // Unlock the bits. bmpScaled.UnlockBits(bmpDataScaled); //set color palette of new image to old image color palette bmpScaled.Palette = bmp.Palette; |
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