If you're a game developer, specifically working on 2d games, you will probably want some sort of graphics sooner or later.
Suffice to say, graphics aren't an easy topic for a programmer to really "get" aside from binary values for R G B A. Yet, for art people, it's all about hues, lighting, perspective, and all that other non-programmer-technical stuff...
Why am I rambling about all of this? Because if you want to incorporate 2d graphics, you will likely need to interact with artists (unless you're the artist, but you should still take heed!), and likewise, they will have to interact with you... and this is where things can get interesting.
Through some miracle, programmers and artists are working together, but how exactly does all this neat 2d art actually work and get in to a game? Texture atlases.
Well, let's rewind a bit, the most basic thing is a single graphic.
Now, how you take a single graphic and display it in a game is dependent on your environment, game engine, and all that other stuff. For the sake of this article, I will not be delving in to any technical examples of how to use any graphics libraries (you won't be seeing any code!) -- I'm more going to focus on *methods* of putting art in to a game.
Now, some programmers might think "methods" and think of programming -- well, reminder, not everyone thinks the same words mean the same things... "methods" can also mean the process of how you accomplish some task... like an artist might use a certain method to create your art, but that has nothing to do with your actual programmatic method used to display that art in your game... or your method of how to implement your method to display the art...
Anyways, back to graphics!
There are some technical issues that you and/or your artist should know right at the get go. First, some terminology.
Raster Graphics - This refers to graphics that are pixel data. Stuff like png files, raw RGBA binary data, bitmaps, things that are defined by pixels. All of this stuff is collectively called "raster graphics". This is as opposed to...
Vector Graphics - These graphics are a bit more complicated and usually not useful to implement directly in to games. However, there are some pretty huge perks to having an artist use this format as opposed to raster graphics. With vector graphics, the artwork is now a collection of mathematical shapes assigned various colors. This means that, like all math, the original will always remain true, regardless of "zoom level" or how much you scale it; unlike raster graphics which use filters to "approximate" when scaling.
The issue of scaling graphics is complex and is worthwhile to research, but the skinny version is that raster graphics lose quality every time you scale up or down from the original until you will just end up with a blob. With vector graphics, you can essentially scale up, down, shear, or do anything else to it without losing quality, as much as you want to.
Now, I mentioned vector isn't directly useful to games. This is true, rendering vector graphics is a massively difficulty task for even today's hardware. Sure, you could render a couple dozen vector graphics, but you need to realize that the graphic being displayed is generated by math, and if that needs to be refreshed (like during a screen redraw!), it could potentially need to be regenerated; so if the shape is complex enough, it will ruin your game's performance.
Ok, now that we got all that out there, it is really only practical to use raster graphics in games! Graphics programming is all about using pixel data to feed directly in to some framebuffer, or use as texels for polygons, or things like this. Someone will need to convert vector in to raster before it can be directly useful in a game. Thankfully, most (all?) vector programs have an export feature and will allow someone to export vector art (which is flawless!) in to a rasterized version of that art. What's even better is that you can essentially export to some given scale right away!
I was once confronted with the idea that an artist could "just give me a really large original" from photoshop and then I could just scale it down based on my need at the time. While this may sound "ok", this is not ok.
Let's talk about graphics scaling some more to hopefully highlight why this isn't acceptable.
We're living in a day and age of thousands of devices, smartphones, tablets, computer monitors, etc. All of them have different display sizes and capacities (what a nightmare!). If you want to deploy your game to some/any/all of them, you will need different sizes for all of your art... meaning a LOT of scaling up and down. But, how do you know what size art you need, or rather, what size would look the best on some given device?
The lazy answer is "just use a big version and scale down", but this actually isn't a good answer at all for 2 reasons. One, you use excessive amounts of memory/cpu/gpu when you scale it in your game engine. Two, the scaled version will look much worse regardless if you did the scaling in-engine or pre-rendered the scaled version (ie, you scaled it down ahead of time in gimp/photoshop).
The correct answer is you have to have an exact pixel match for your art to any given display size (read: not feasible).
Anyways, if those 2 lazy reasons don't scare you away, then consider this: scaling graphics up or down too much will always look bad regardless of the originals' size. I believe the magic cutoff is about 50% up or down before it really starts looking terrible.
Example time. Let's say I have a 5000x5000 original, which should be considered excessive. Yet, it does fit the artists' idea of a very large original, and if we can just scale it down to fit our needs, what's the problem as long as we scale it down before we actually use it?
Think like an artist here. a 5000x5000 original will look absolutely amazing, and the art going in a game should look amazing, so what's the problem? Well, aside from the obvious memory issue (5000x5000 = 25 million pixels, at 4 bytes per pixel... 100 million bytes to even open that graphic in game!), the appearance will be very terrible if you have to scale it down to something like 128x128. Go ahead and try it in your favorite raster art program, make a doodle at 5000x5000 and scale it to 128x128 and look what happens, especially around the edges!
Now, this issue is actually more problematic, not only will this scaled version look horrendous, it will actually likely get scaled again when you go to display it in game! A scaled version of an already bad scaled version will look catastrophic at best.
We could take some less absurd examples, but hopefully you get the idea that a scale of a scale is bad... with larger scaling happening anywhere meaning exponentially bad results.
Vector art sort of fixes this (it's not a panacea though). The original export will get you a more or less "perfect" raster version at your specified dimensions during the export process. There isn't any scaling of a raster, there is just a mathematically generated piece of art. This doesn't work too well with very small graphics (like exporting to say under 32x32ish), so if you're dealing with very small graphics, it is probably best to start doing pixel art.
This generated/exported art will still probably get scaled when it comes time to use in your game and this is the reason why you will likely want various versions of each art asset for different display densities (which is a whole other topic). Long story short though, you will want to export new originals from the vector art at different pixel sizes rather than scaling your already generated raster art. The result will likely be much sharper looking art in the end product! I said likely because there are other steps that can mess up your art all along the way.
Next article I'll cover more about finally putting some of these graphics to work and texture atlases.
Check out part 2
Showing posts with label libpng. Show all posts
Showing posts with label libpng. Show all posts
Wednesday, May 8, 2013
Thursday, March 29, 2012
Android NDK loading resources and PNGs
After following the nice code sample from http://androgeek.info/?p=275 about how to load a PNG file by accessing the Android APK file directly using only the NDK, everything seemed awesome.
It uses libzip and libpng to manually load the APK, then load a png file right from the APK itself without using any Java or the Android SDK.
Why would you want to do this? Well, aside from liking complicated stuff, it is useful for us because now we can put all of our "portable" resource loading code in c/c++ files so it is more easily moved to iOS, windows, whatever, without being bound to using only the Android SDK, thus having to recode loading logic over and over again for each target deployment architecture.
Everything worked well, until something didn't go quite right. Some of my textures were corrupted and worse, some actually crashed my app!
It took 4 days to track down what exactly was happening.
Common problems usually indicate I was doing something wrong with opengl and the textures themselves...
* All my textures were perfect powers of two (64x64, 128x128, etc)
* All my textures were 32-bit RGBA files
* I fixed a warning and error from the given sample code to correctly read the libzip'd apk file
So, it was time to dig deeper with what was going on.
First, I noticed that my glGenTextures calls were giving me very large opengl texture ids so I thought this was a huge problem as it was not continuous numbers, and in C, any time you have generating wild numbers, it usually means a bad pointer somewhere. I spent about a day analyzing and referencing code and started to realize that the generated numbers were always the same (this usually rules out a memory corruption issue if the results are *always* the same).
I noticed that the generated opengl texture ids always followed this pattern on my Droid RAZR:
100271
315638026
534244737
1505553200
-1563003837
...
etc.
Yet, on my Android emulator, the texture id's were:
1
2
3
4
5
...
etc
I had found this thread on google groups that I thought was my problem since it sounded really similar, but it didn't actually help.
Knowing that glGenTextures expects an *unsigned* integer as the data type, I realized that the negative number was an overflowed signed integer and actually nothing to worry about. In my debugging print code I needed to change "%d" to "%u" to see the actual unsigned value.
Since these numbers repeated on every subsequent run of the program, I decided to test the idea that maybe my code had nothing at all to do with the "corrupt" large texture ids. Indeed, I made a program that literally only called glGenTextures about 100 times right at startup time and did nothing else. The result? The exact same large numbers showed up in my device, and was "normal" on the emulator, as expected. This left me with the conclusion that whatever logic is in my Motorola device in terms of picking a unique texture ID that this was normal - next.
Yet, this didn't answer my question as to why I was seeing an app crash occur and seemingly only when loading a certain texture or two. My first order of business was to rule out if my other code was at fault, or was it somehow related to ONLY those sets of textures (somehow). Strangely, dozens of tests indicated that it was only the problem of 3 specific textures being loaded as I added lots of additional textures, even loaded some textures multiple times, then finally ONLY loading one of those 3 problematic textures. The app would only crash at loading any of the 3.
Frustrated, I ended up putting a ton of extra debugging code all over my apps trying to track down what in the world was wrong. All indicators were pointing to the idea that a problem was with either libzip or libpng since libc.so itself was crashing in a call to a libpng function -- great, time to track down potential bugs in those libraries.
Now, let me put a disclaimer up here, it is almost never a good idea to assume a mature code library has a bug in it; especially one this severe, yet here I am with a consistent crash always calling a specific line of code that libpng was using leaving me to believe this could be the case.
First, I downloaded the most recent version of each library and replaced the old versions that were provided in the linked post above. I was now using libzip 0.10.1 and libpng 1.5.9. This had promise since each of those had a bit of fixes/enhancements along the way. I was hopeful that this would magically fix my problems.
Well, it did - sort of.
Whatever wizardry was happening internally in those libraries helped indicate that libpng was crashing specifically on this line:
png_read_info(png_ptr, info_ptr);
Great, all this work only to help reinforce the idea that libpng was broken for my special case. Yet, I still couldn't rule out that libzip somehow corrupted my image data while decompressing my APK so libpng might not be at fault trying to load corrupted data. Worse, all error checking came up empty for all libzip and libpng calls! I even checked my opengl calls for errors using glError -- NOTHING!
Well, the next step was to dive in to those library's source code and start tracking stuff down, the last resort I guess was to find the problem in one of these libraries and fix it myself and give the respective team a patch!
One of my testing tangents included making a stand alone C project to test very specific sets of code to completely rule out any Android Java JNI mysticism causing problems. Everything was pointing to only those textures being a problem in the libpng loading code. So I ran the libpng test program on those 3 test pngs and the only thing that sounded remotely threatening was:
Files aaaa.png and pngout.png are different
Was aaaa.png written with the same maximum IDAT chunk size (8192 bytes), filtering heuristic (libpng default), compression level (zlib default), and zlib version (1.2.5)?
It certainly sounded menacing and I was convinced that somehow my textures were corrupted. I ended up regenerating those textures using the latest version of Gimp (2.6.11 at the time) -- plugged those in to my app in excitement; and they still crashed.
Strangely, the libpng test program generated pngout.png which was seemingly the same exact copy of my original texture, but a different filesize; something was different, and different was good at this point. For giggles, I put that pngout.png file in to my Android's APK res/raw folder in place of my original texture and much to my surprise the texture loaded!!! Ah-hah, the answer finally!
Unfortunately, things weren't over yet, the texture loaded completely corrupted. Everything was misaligned, the colors were wrong and the texture was completely garbled with random colors all over it. Basically, I've gone nowhere.
3 days in and I'm still struggling with what is wrong here. I'm still baffled, why is it that my 10 other textures work fine, yet these 3 don't want to work at all?! I decided to write my own texture loading function from scratch to ensure I understood exactly every step of logic that was taking place. The same result happened. I did all sorts of strange additional tests, hex dumps, you name it. I was at the end of my rope on this one, I was getting angry.
Finally, I saw an interesting article somewhere that said I needed to rename my png files to mp3 files. I thought to myself, that's the dumbest thing I ever heard - why would I ever want to do that?! Well, if you're programming for Android, you might be crazy enough to hear this out.
Apparently, during the APK packing process *SOME* of your png textures are automagically compressed and others are not. Renaming your png files tells the apk packing process to not mangle/compress that file, and my happy 3 textures were some of the lucky selected ones for this process; which completely broke the texture loading code. In a move of desperation, I did rename my files to mp3 to verify this was the case, and they worked perfectly. I was enraged and ecstatic at the same time.
On the bright side I got a better understanding of both libpng and libzip (we got a little familiar with each other, if you know what I mean).
Alright, so the magic bullet answer is that I need to rename all my PNG files to MP3 (test.png.mp3 if you will) or simply dump them all in to a game data file of sorts (probably the better idea).
So, I make a quick resource packing program (using zlib) and I find out that during the APK packing process again, my GZ files are also modified!!!
I quickly found this link explaining it a little bit.
I wonder what other goodies are in store for me to discover by accident!?
Hope this article is found by anyone else stuck in my situation and save you all a couple days of madness.
It uses libzip and libpng to manually load the APK, then load a png file right from the APK itself without using any Java or the Android SDK.
Why would you want to do this? Well, aside from liking complicated stuff, it is useful for us because now we can put all of our "portable" resource loading code in c/c++ files so it is more easily moved to iOS, windows, whatever, without being bound to using only the Android SDK, thus having to recode loading logic over and over again for each target deployment architecture.
Everything worked well, until something didn't go quite right. Some of my textures were corrupted and worse, some actually crashed my app!
It took 4 days to track down what exactly was happening.
Common problems usually indicate I was doing something wrong with opengl and the textures themselves...
* All my textures were perfect powers of two (64x64, 128x128, etc)
* All my textures were 32-bit RGBA files
* I fixed a warning and error from the given sample code to correctly read the libzip'd apk file
So, it was time to dig deeper with what was going on.
First, I noticed that my glGenTextures calls were giving me very large opengl texture ids so I thought this was a huge problem as it was not continuous numbers, and in C, any time you have generating wild numbers, it usually means a bad pointer somewhere. I spent about a day analyzing and referencing code and started to realize that the generated numbers were always the same (this usually rules out a memory corruption issue if the results are *always* the same).
I noticed that the generated opengl texture ids always followed this pattern on my Droid RAZR:
100271
315638026
534244737
1505553200
-1563003837
...
etc.
Yet, on my Android emulator, the texture id's were:
1
2
3
4
5
...
etc
I had found this thread on google groups that I thought was my problem since it sounded really similar, but it didn't actually help.
Knowing that glGenTextures expects an *unsigned* integer as the data type, I realized that the negative number was an overflowed signed integer and actually nothing to worry about. In my debugging print code I needed to change "%d" to "%u" to see the actual unsigned value.
Since these numbers repeated on every subsequent run of the program, I decided to test the idea that maybe my code had nothing at all to do with the "corrupt" large texture ids. Indeed, I made a program that literally only called glGenTextures about 100 times right at startup time and did nothing else. The result? The exact same large numbers showed up in my device, and was "normal" on the emulator, as expected. This left me with the conclusion that whatever logic is in my Motorola device in terms of picking a unique texture ID that this was normal - next.
Yet, this didn't answer my question as to why I was seeing an app crash occur and seemingly only when loading a certain texture or two. My first order of business was to rule out if my other code was at fault, or was it somehow related to ONLY those sets of textures (somehow). Strangely, dozens of tests indicated that it was only the problem of 3 specific textures being loaded as I added lots of additional textures, even loaded some textures multiple times, then finally ONLY loading one of those 3 problematic textures. The app would only crash at loading any of the 3.
Frustrated, I ended up putting a ton of extra debugging code all over my apps trying to track down what in the world was wrong. All indicators were pointing to the idea that a problem was with either libzip or libpng since libc.so itself was crashing in a call to a libpng function -- great, time to track down potential bugs in those libraries.
Now, let me put a disclaimer up here, it is almost never a good idea to assume a mature code library has a bug in it; especially one this severe, yet here I am with a consistent crash always calling a specific line of code that libpng was using leaving me to believe this could be the case.
First, I downloaded the most recent version of each library and replaced the old versions that were provided in the linked post above. I was now using libzip 0.10.1 and libpng 1.5.9. This had promise since each of those had a bit of fixes/enhancements along the way. I was hopeful that this would magically fix my problems.
Well, it did - sort of.
Whatever wizardry was happening internally in those libraries helped indicate that libpng was crashing specifically on this line:
png_read_info(png_ptr, info_ptr);
Great, all this work only to help reinforce the idea that libpng was broken for my special case. Yet, I still couldn't rule out that libzip somehow corrupted my image data while decompressing my APK so libpng might not be at fault trying to load corrupted data. Worse, all error checking came up empty for all libzip and libpng calls! I even checked my opengl calls for errors using glError -- NOTHING!
Well, the next step was to dive in to those library's source code and start tracking stuff down, the last resort I guess was to find the problem in one of these libraries and fix it myself and give the respective team a patch!
One of my testing tangents included making a stand alone C project to test very specific sets of code to completely rule out any Android Java JNI mysticism causing problems. Everything was pointing to only those textures being a problem in the libpng loading code. So I ran the libpng test program on those 3 test pngs and the only thing that sounded remotely threatening was:
Files aaaa.png and pngout.png are different
Was aaaa.png written with the same maximum IDAT chunk size (8192 bytes), filtering heuristic (libpng default), compression level (zlib default), and zlib version (1.2.5)?
It certainly sounded menacing and I was convinced that somehow my textures were corrupted. I ended up regenerating those textures using the latest version of Gimp (2.6.11 at the time) -- plugged those in to my app in excitement; and they still crashed.
Strangely, the libpng test program generated pngout.png which was seemingly the same exact copy of my original texture, but a different filesize; something was different, and different was good at this point. For giggles, I put that pngout.png file in to my Android's APK res/raw folder in place of my original texture and much to my surprise the texture loaded!!! Ah-hah, the answer finally!
Unfortunately, things weren't over yet, the texture loaded completely corrupted. Everything was misaligned, the colors were wrong and the texture was completely garbled with random colors all over it. Basically, I've gone nowhere.
3 days in and I'm still struggling with what is wrong here. I'm still baffled, why is it that my 10 other textures work fine, yet these 3 don't want to work at all?! I decided to write my own texture loading function from scratch to ensure I understood exactly every step of logic that was taking place. The same result happened. I did all sorts of strange additional tests, hex dumps, you name it. I was at the end of my rope on this one, I was getting angry.
Finally, I saw an interesting article somewhere that said I needed to rename my png files to mp3 files. I thought to myself, that's the dumbest thing I ever heard - why would I ever want to do that?! Well, if you're programming for Android, you might be crazy enough to hear this out.
Apparently, during the APK packing process *SOME* of your png textures are automagically compressed and others are not. Renaming your png files tells the apk packing process to not mangle/compress that file, and my happy 3 textures were some of the lucky selected ones for this process; which completely broke the texture loading code. In a move of desperation, I did rename my files to mp3 to verify this was the case, and they worked perfectly. I was enraged and ecstatic at the same time.
On the bright side I got a better understanding of both libpng and libzip (we got a little familiar with each other, if you know what I mean).
Alright, so the magic bullet answer is that I need to rename all my PNG files to MP3 (test.png.mp3 if you will) or simply dump them all in to a game data file of sorts (probably the better idea).
So, I make a quick resource packing program (using zlib) and I find out that during the APK packing process again, my GZ files are also modified!!!
I quickly found this link explaining it a little bit.
I wonder what other goodies are in store for me to discover by accident!?
Hope this article is found by anyone else stuck in my situation and save you all a couple days of madness.
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