How can I change all pixel values of a portion of an image to a certain value?

Question

Consider an image (img) as

What is the best way to convert all the pixel values to say, 0.8?

Edit 1 Let me explain my objective. Firstly, I partition the image into 4 parts.

{width, height} = ImageDimensions[img];
Flatten@ImagePartition[img, {225/2, 225/2}];

Now I want to change the pixel values of the first image part to be 0.8 and then merge all the parts together. One thing to be noted here is that I don't have the information about the pixel values in the parts and all the pixels may not have the same value.

One issue in changing the pixel value in the part image is that it may create artifacts in the boundary while merging them together after changing pixel value. Therefore, I need a way such that there be a mapping between the first part and the corresponding portion in the original image so that I can make the changes directly on the image instead on the part.

Edit 2 I have applied image partitioning to treat individual parts as blocks so that I can compare the blocks with its neighbor to set the condition for changing the pixel value in a block.

Answer 1

MapAt can be used:

MapAt[ImageAdd[ImageMultiply[#, 0], 0.8] &,
   ImagePartition[img, {225/2, 225/2}], {1, 1}]

Answer 2

not sure why you want to do the partitioning. ReplacePixelValue can do it direct.

img = ImageResize[ExampleData[{"TestImage", "Mandrill"}], 100]
ReplacePixelValue[img, {1 ;; 50, 1 ;; 50} -> Hue[.3]]

actually partitioning proves to be considerably faster..

part = ImagePartition[img, {50, 50}];
ImageAssemble[
 MapAt[Image[ConstantArray[Hue[.3], ImageDimensions[#]]] &, 
  part , {2, 1}]]

Answer 3

The two functions it sounds like you want to use are ImageApply and ImageApplyIndexed.

Let's make a random base image:

baseIm = RandomImage[];

Now, we want to turn the right half to 0.8-valued pixels.

ImageApply will get all of them. To choose, we build a mask image.

mask = ImageAssemble[{ConstantImage[0, {75, 150}], ConstantImage[1, {75, 150}]}];

Now, we can turn every pixel selected by the mask to 0.8 (or apply whatever function we want to it):

ImageApply[0.8 &, baseIm, Masking -> mask]

If we already know which pixels we want and don't want to go to the trouble of building a mask, we can use ImageApplyIndexed, which gives position as an argument to our function.

Let's make a random base image:

baseIm = RandomImage[];

Now we apply a function that makes the pixel value 0.8 if we are to the right or below the mid-lines:

ImageApplyIndexed[If[#2[[1]] > 75 && #2[[2]] > 75, 0.8, #1] &, baseIm]