Why different size of the PNG-24 file format: vs deletion of pixels in the layer mask

I noticed in Photoshop CS5.1 and CS6 strange behavior when saving web files in the PNG-24 format.

CASES1.

I have a layer photo 260 x 360.

Create square 200 x 200 and layer mask to hide selection (effect: transparent square hole inside)

Save for web in format PNG-24 (transparency)-> 131.4 k file size

CASE2.

I have a layer photo 260 x 360.

Create the square Marquee 200 x 200 and delete pixels (effect: transparent square hole inside)

Save for web in format PNG-24 (transparency)-> 66,93 k file size

Both pictures got same transparent square hole, but sizes are so different. Using mask does not reduce the size of the file PNG24. Why is it like that, bug?

mask.pngcutout.png

For cases 1 and 2 the number of pixels in the image is exactly the same thing then you must consider other factors to take into account the file size differences.

Assuming that the image is a RGB/8 image:

1 uses a mask which means that it is a picture of four channels that require 32 bits per pixel.

Case 2 has no mask is only a picture of 3 channels that require 24 bits of pixel.

If the content of the data in Photoshop is different for the two cases, and the psd for case 1 file size would be larger than in case 2.

In PNG, transparency of files can be specified by declaring the individual pixels to be transparent or an alpha channel can be added. Again, it is 24 bits per pixel compared to 32 bits per pixel.  If Photoshop, using enregistrer save for Web, creates an alpha channel for cases 1 and says the individual pixels to be transparent in the case 2, then the differences in file size that you can find should.

Paulo

Tags: Photoshop

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