Sampling and subsampling

• Method: Average downsampling reduces the resolution of an image by averaging the color values of multiple pixels into a single pixel. For example, if you are downsampling by a factor of 2, a 2x2 block of pixels in the original image is averaged to produce one pixel in the downsampled image.

• Quality: This method tends to produce smoother and higher-quality images because it takes into account the color information of all the pixels in the block, resulting in less aliasing and more accurate color representation.

• Use Case: Average downsampling is often used when maintaining image quality is important, such as in high-quality print documents.

• Method: Subsampling reduces the resolution by simply selecting one pixel from a block of pixels and discarding the rest. For example, if you are subsampling by a factor of 2, a 2x2 block of pixels in the original image is reduced to one pixel by selecting one of the four pixels and ignoring the others.

• Quality: This method can result in lower image quality because it does not consider the color information of the discarded pixels, which can lead to aliasing and loss of detail.

• Use Case: Subsampling is often used when file size is a higher priority than image quality, such as in web documents or when bandwidth is limited.

In the context of optimizing a PDF file, the choice between average downsampling and subsampling depends on the desired balance between image quality and file size.

• Average Downsampling: Produces higher-quality images by averaging pixel values, resulting in smoother images with less aliasing.

• Subsampling: Produces lower-quality images by selecting one pixel from a block and discarding the rest, resulting in faster processing and smaller file sizes but with potential loss of detail and increased aliasing.