Can a PNG file's data be loaded in a certain order, say beginning with one RGBA channel, then the rest?
Perhaps similar to how JPG files can, sometimes using custom codecs, store their data as "scans" or "slices", that are loaded progressively, sequentially or interleaved, at first displaying black and white, followed by red, green, and finally blue.
It seems very unlikely, but can PNG files have a way of behaving like this?
References:
https://cloudinary.com/blog/progressive_jpegs_and_green_martians
Besides standard/progressive, the 3rd kind of JPEG compression: load by channel?
https://graphicdesign.stackexchange.com/a/55570/97317
PNG's equivalent to JPEG's progressive display is interlacing. Rather than interlacing lines, it interlaces pixels using a pattern the PNG standard calls ADAM7.
Related
I have an image with grayscale background and 2 thin lines (1 pixel wide) drawn in color. I'm trying to use various JPEG compression table (luminance and chrominance) to get the best possible result while staying under a certain file size.
The grayscale background compresses well and looks decent. The thin vertical and horizontal color lines get mutilated / smeared. The current JPEG algorithm uses 2x2 sub-sampling on the Cb and Cr channels and the chrominance compression table is fairly aggressive (high compression).
Is there any way to embed a "BMP" type data into JPEG image. Basically specify a color for specific pixels to be applied after the JPEG is de-compressed?
Any other ways clean up how thin color lines get encoded / decoded in JPEG without increasing the overall file size by a lot.
P.S. I'm testing all this stuff in Matlab.
The image at the end of this question is a PNG with mode I, which stands for Indexed, as far as I can tell.
I'm trying to create a thumbnail out of it, and save it as JPG with PIL.
However, is I leave the mode alone, PIL won't let me resize it with error unable to generate thumbnail: cannot write mode I as JPEG.
If I convert it to RGB, the result will be a fully white image.
Is there a way to fix this?
https://www.dropbox.com/s/2d1edk2iu4ixk25/NGC281.png
The input image is a 16-bit grayscale PNG, and it appears PIL has a problem with this. Manually converting it to an 8-bit image before further processing makes it work again.
The problem may originate inside PIL itself. The PyPNG homepage asserts
..PIL only has internal representations (PIL mode) for 1-bit and 8-bit channel values. This makes me wonder if PIL can read PNG files with bit depth 2 or 4 (greyscale or palette), and also bit depth 16 (which PNG supports for greyscale and RGB images).
Then again, that page is from 2009. It could be worth tracking down where PIL is maintained from, and report this as a bug (? Or possibly a feature request?).
JPEGs are smaller in size than PNGs. So, I thought that if I can make a specific region in a JPEG-file transparent, with some code, maybe I can save some bytes.
So does anyone know how to achieve this with for example PHP or JavaScript?
No. You can't do this. JPGs do not support alpha channels and have no capacity to designate certain colors as transparent either (GIF-style).
There's several issues with this, all of them have to do with that JPEG is a lossy compression format. The JPEG format is optimized for natural images and sharp edges will get blurred. If you intend that a specific pixel should have the value #d67fff there's no guarantee that after color conversion, FDCT, quantization, IDCT and color conversion, the pixel still will have that value. There's also a strong possibility that that pixel value will occur in areas that you don't want.
No. JPEG does not support transparency and is not likely to do so any time
soon. http://www.faqs.org/faqs/jpeg-faq/part1/section-12.html
You cannot do that, the client renders the image and doesn't know that you want it to treat that color as transparent (plus various compression methods on jpeg wouldn't work well with transparencies anyway).
I believe you can go with an 8-bit custom-pallet png, should save you a lot of space. Otherwise 24-bit PNG is your only high color option.
You can convert your image to SVG containing a color information as JPEG and an alpha channel as grayscale mask. Here is a tool I wrote to do it https://github.com/igrmk/transpeg
I 'd like to prepare my PNGs for the best optimization, so I can get the best image quality (lossless if possible) and the smallest size.
From what I understand, I should use: PNG, 72 dpi, RGB, but what else?
Here is what we find in the iPhone HIG:
Note:*The standard bit depth for icons and images is 24 bits (8 bits each for red, green, and blue), plus an 8-bit alpha channel. The PNG format is recommended, because it preserves color depth and supports an embedded alpha channel.
I guess this mean we should save the image as PNG 24 and create them in 8 bits mode? But I also read about 32 bits for best quality ?
The interlacing scheme (witch add to the file size) allows for the PNGs to display faster. Does this applies to the iPhone?
Thanks.
I suggest using ImageAlpha (lossy) on as many images as you can, because it greatly reduces their size.
Optimize all images with ImageOptim — it will remove all invisible junk and re-compress the data.
Disable Xcode conversion, because it undoes other optimisations and can make images much slower to load.
24 bit is red, green and blue with 8 bits each. 32 bits is RGB plus an 8-bit alpha channel. So if you need (semi-)transparent images, you should go for 32bit PNG, otherwise 24bit.
You don't have to compress/crush the PNGs yourself, Xcode's build steps will automatically use pngcrush and re-order the color channels for the iPhone's BGR memory alignment.
For my background app I am using JPEG (Export for web in photoshop) with quality 70.
Last day I heard about pngcrunch, tried it but file size is the same...
http://pmt.sourceforge.net/pngcrush/
Take a look at this previous post :
Understanding 24 bit PNG generated with Photoshop
I am converting eps (Encapsulated PostScript) files to jpeg files with ghostscript. A sample command I use is:
gswin32.exe -sDEVICE=jpeg -dJPEGQ=100 -dNOPAUSE -dBATCH -dSAFER -r600x600 -dGraphicsAlphaBits=4 -dUseCIEColor -dEPSCrop -sOutputFile=”a.jpeg” b.eps
The input eps files come with white backgrounds (I only have their clipping path). What I need to do is change this white background to another color in the output images, or it would be even better if I could make them transparent (output file format would be png). How can I do this?
never tried it myself but you should be able to convert your eps file into png by setting:
-sDEVICE=pngalpha
also the pngalpha device has a -dBackgroundColor option:
-dBackgroundColor=16#RRGGBB (RGB color, default white = 16#ffffff) For
the pngalpha device only, set the
suggested background color in the PNG
bKGD chunk. When a program reading a
PNG file does not support alpha
transparency, the PNG library converts
the image using either a background
color if supplied by the program or
the bKGD chunk. One common web browser
has this problem, so when using on a web page you
would need to use
-dBackgroundColor=16#CCCC00 when creating alpha transparent PNG images
for use on the page.
more details here: Details of Ghostscript output devices see section 3.1. PNG file format
After you've obtained your (white background) images from Ghostscript, you could use ImageMagick's convert or GraphicMagick's gm convert commands to change the white to transparent background:
convert -background transparent my.png my_transp.png