The Power Of In-App Purchases

I finally managed to get through the hotel wifi and upload the slides for this morning’s 360iDev talk: The Power Of In-App Purchases. Thanks everybody who attended for the great questions and feedback!

Session description

The common-sense approach to make money on the App Store used to be to do anything to get on the top charts. In-app purchases changed all of that. Good in-app purchases can make your app profitable without being anywhere on the charts, and are the best hope for the independent developer. Come to this session to learn why IAPs can be so effective and how to leverage them effectively: what makes a good IAP, how to increase your user involvement, how to present IAPs in an attractive way, what things attract users, and what things turn them away. We’ll go through lots of detailed real-world data from Flower Garden and other games with strong IAPs.


Presentation slides: [Slideshare] [pdf]

Chronicle Of A Failed Experiment

In the last year and a half, I’ve written about the different things that I’ve tried with Flower Garden and their effects on sales. From adding Facebook support, creating a free version, adding in-app purchases, or giving Flower Garden for free for a limited time. Some strategies worked and some didn’t.

Today I want to share my latest experiment, and how it was a total and complete failure: Adding the option to gift IAP items from within the game.


Promo Codes

Before I can talk about how I implemented the “Gift This” feature, we need to talk about promo codes. Since Flower Garden has so many in-app purchases, I figured it would be very handy to have the ability to give some of them away to people for any reason: They mistakenly downloaded the wrong thing, they bought it in the free version and want to upgrade to the full one, or they supposedly paid for one but they never got it. Whatever the reason, having my own promo codes for in-app purchases was one of the best decisions I could have made. I would highly recommend it if you have IAPs, especially consumables.

The implementation of promo codes was totally straightforward. I have two tables in the Google App Engine: One for active promo codes and one for redeemed ones. The active promo code includes the code itself, the IAP item it refers to, the amount, and whether it’s limited to one user or not (if it’s limited to one user, the code goes away as soon as it’s redeemed, otherwise, any amount of users can redeem it).

Here’s an example of a code I just added (yes, feel free to redeem it in the Flower Shop):


Whenever a promo code is redeemed, I enter that data in the other table. That way not only do I have a log of what codes where redeemed and when, but I can also check and prevent the same device from redeeming the same code multiple times.

Another important reason to keep a redeemed promo code table is that I want promo codes to be as valid as purchasing the IAP directly. That means that if you ever attempt to purchase an item, I check first to see if you’ve redeemed a promo code for it, and if so, you can re-download it for free. Same thing when you do a restore purchases (although I believe I haven’t gotten around to implementing that part yet 🙂

Here’s my plea to Apple: Please, please, please, give us an “iTunes Account ID” along with the IAP data. Right now the best we can do is associate a purchase with a device (which is not good enough), or have a whole registration system for users (which is a pain and more time consuming for users). They’re already doing this with a Game Center ID, so why not with an iTunes Account?

Gift This

Once the promo code system was in place and field tested for a couple of months, I finally implemented the Gift This functionality. In the Flower Shop, users have the option to buy an item for themselves, or for someone else.

The first time you use the Gift This feature it explains how it works: You purchase the item and then you send it to someone else through email.

Under the hood, it purchases the IAP, contacts the server to generate a new promo code for that item, and then creates an email with the promo code and even a custom URL. Whenever someone receives a gift email, they can just click on the custom link and they immediately receive the item (assuming they have Flower Garden installed, of course).


One interesting consequence is that to implement this, you need to create one new IAP item for every item you want to gift (especially if they’re not consumable). Otherwise, someone couldn’t gift an item they had already purchased, or they couldn’t gift it more than once. This can add quite a few extra IAPs in your list!

In the case of Flower Garden, I started with the easiest case, and I only implemented gifting for fertilizer purchases (because they’re consumable, so I don’t have to keep track of who receives them and restoring them).

Total Failure

I really had great hopes for the Gift This feature. I had already envisioned writing a blog post showing IAP revenue going up 20-30% because of that feature. Not quite! It was pretty much a complete flop. The only positive thing I can say about it is that it didn’t actually lower regular sales.

See for yourself. In a period of about a month and a half, all the fertilizer gifting in the free and paid versions of Flower Garden amounted to a whopping $191!


Definitely not worth the 3-4 days it took me to implement it (and the opportunity cost of not being able to add some other feature or IAP).

Compare it to fertilizer sales during that period of over $7,000:


(That’s the spike of the new feature plus the Pocket Frogs cross promotion if you were wondering about it).

I’d love to know how the Gift This feature on the App Store is working out for Apple. I’m sure it’s doing better than my attempt at it, but I’m going to bet it’s still a very small percentage compared to regular sales.

Here comes the important question: Why was it a failure? Do people don’t like to gift? Was it presented badly? Did most people not know it existed?

There’s no way to know for sure, but my current guess is that people don’t like to gift something they don’t already own. Psychologically, there are several too many steps involved: Oh, I want to gift something, look for it in the store, purchase it, and send the email. Not a very fulfilling experience.

On the other hand, gifting something you already own is much more appealing. You have it in front of you, you’re proud of it and it looks great. You tap on a button and send it to someone. That is a lot more satisfying. So in the future I’ll take that approach and allow people to gift things they already own, even if they had to previously pay for it in some way.

What do you think? Do you have a better theory? How could it have been improved?

This post is part of iDevBlogADay, a group of indie iPhone development blogs featuring two posts per day. You can keep up with iDevBlogADay through the web site, RSS feed, or Twitter.

Life In The Top 100 And The Google App Engine

A few weeks ago I wrote about using the Google App Engine as a back end for Flower Garden. One of the comments I heard from several people is that the Google App Engine pricing scheme is “scary” due to its unpredictability. What if your server gets very busy and you find yourself with a huge bill?

Google App Engine Costs

At the time I wrote that post, the bandwidth of Flower Garden (and Flower Garden Free) was just under the free bandwidth allowance, so I didn’t have to pay anything. However, this last week, after riding the Pocket Frogs rocket, Flower Garden Free shot up to the #56 overall app in the US. And boy, did that make a difference in server usage!


That screenshot shows the Google App Engine dashboard with the number of requests per second for a 4 day period. Can you guess when the Pocket Frogs cross-promotion started? It went from an average of 1.5 requests per second, to a peak of about 16. That’s a factor of 10 right there.

Let’s see what that means in terms of cost. With the Google App Engine, you get charged for different resources you consume: CPU usage, incoming bandwidth, outgoing bandwidth, data storage, and recipients emailed. Of those, the only one that Flower Garden makes any dent on is outgoing bandwidth (mostly the HTML pages and images in the Flower Shop).

For each of those resources, you get a free amount every day. In the case of outgoing bandwidth, we get 1GB before we have to start paying anything. Afterwards, it’s $0.12 per GB. The peak day for Flower Garden Free, when it was all the way at #56, it consumed 7.8 GB. Now that it’s back down, it’s using up at around 2-3 GB of outgoing data every day.

That means, that the busiest day I was charged $0.82 in bandwidth, but the profits for that day were over $1500! I can only hope for days just as busy!


As a reference, these are my Google App Engine charges for the busy week during the promotion. $3.28 for the week? Bring it on! 🙂

Minimizing Bandwidth

When I first saw the first day’s bandwidth, I was pretty surprised it was taking up that much. I hadn’t tried to minimize it first, but 8 GB seemed like a significant amount.

The first thing I did was to forward any requests to the “More Games” and “News” section back to the Dreamhost account. After all, if that server goes down, it doesn’t really matter, and they can take up significant bandwidth with all the images.

To forward something, I couldn’t just do it from the configuration file. Instead, I set a handler for the moregames directory like this:

- url: /moregames/.*

And the redirect script is very simple:

import os
import sys
import logging
from google.appengine.ext import webapp
from google.appengine.ext.webapp.util import run_wsgi_app

class RedirectMoreGames(webapp.RequestHandler):
	def get(self, params):
		url = ""
		self.redirect(url, permanent=True)
application = webapp.WSGIApplication(
							[(r'/(.*)', RedirectMoreGames),

def main():

if __name__ == "__main__":

Another thing you can do is to turn on explicit caching of files in the app.yaml file for your Google App Engine app. I don’t know how much that helps with data requested from within an iPhone app, but it can’t hurt.

Finally, I made some changes to Flower Garden. I never had bandwidth minimization in mind, so for example, I was happily requesting updated news and other data at the beginning of every session. I wised up a bit and now it only requests that data at most once every few hours or days.

In case you’re ever getting close to using up your quotas, I learned that you can query them at runtime. I haven’t done anything about it yet, but I suppose I could start forwarding image fetches to a backup server whenever I get close to the limit. But for that, I’ll have to sell many more units of Flower Garden!

Google App Engine Shortcomings

Overall, I’m very happy with the Google App Engine. But it’s not all a bed of roses over here. I think the Google App Engine excels at scaling under heavy load, but surprisingly, it doesn’t have anything close to 100% uptime. I’m going to say it probably doesn’t even have 95% uptime! With such a massive system and all the redundancy underneath, I’m surprised they can’t have better uptime. Supposedly their status page makes you think everything is perfect, but it’s really far from it.

Apart from downtime, people are also reporting severe latency issues sometimes, and I’ve seen some errors like that in my log every so often. Chalk it up to being a beta I suppose. I imagine it’s only going to get better with time.

I would still use the Google App Engine for any new game I’ll release in the future, whether it has big or small backend needs. If nothing else, the ease of development and testing beats every alternative for me.

This post is part of iDevBlogADay, a group of indie iPhone development blogs featuring two posts per day. You can keep up with iDevBlogADay through the web site, RSS feed, or Twitter.

Communicating With Players

By now every iOS developer knows that making a great game and putting it on the App Store is only part of the work. In order to get significant sales, it needs to be noticed. You need to spend a significant amount of time in marketing and PR, making sure that blogs cover it, magazines review it, or at least jump-starting it with a group of devoted and vocal forum fans.

Most often, the advice stops there. So you get your initial sales spike and then sales go way down. What do you do then? Usually, developers release new features and updates. That’s great, but how do you get people to notice it. You need to establish some form of communication with your players.

Update Messages

The simplest form of communication is through the “What’s new” section in the update. You can use that section not just to list what features you added and what bugs you fixed, but also to let your players know about other things: plans for the future, other games to try, or even the URL for your Facebook group.

Credit goes to Igor for bringing up this idea and pointing out the URLs are even clickable in this field (but they aren’t in the app description).

Of course, if you’re like me and you update your apps only once in a while, this technique isn’t as effective. Right now my iPhone tells me I have 67 new updates available. I’m clearly not going to be reading through the release notes of each one.

In-Game News

fg_promo.jpgA more direct way of reaching out to your players is to have some sort of in-game news system. At any point you can update a file in your web server with any news, and it is displayed in the game. It can be implemented in many different ways depending on “on your face” you want to be about it: a pop up that comes up when the user runs the game, a ticker that runs constantly across the bottom of the screen, or, what I did for Flower Garden, a news icon with a badge indicating how many unread items there are, that brings up the news page when you tap it.

Make sure players are able to click on URLs in your news messages so you can direct them to different web pages easily. More on that on a bit.


Update messages are only good for players who update their apps (ahem, ahem), and in-game news for active players who’re currently launching your apps. For maximum effect, you can send an email newsletter and that way you can also reach users who played the game at some point in the past, but aren’t currently playing it now. They’re the ones probably most interested in new updates and features, and chances are you can rekindle their interest in the game.

To do this, you should encourage users to register for your mailing list, or collect their emails with their consent in some other way. This was a technique I started using back in December of last year with great success.

I’m currently using Your Mailing List Provider as a means to delivering thousands and thousands of emails [1]. By the way, don’t miss a chance to join the Flower Garden mailing list 🙂


Similar to the mailing list approach, Facebook groups can be a very effective form of communication. An additional benefit is that friends of your players might see them participating in the page and might make them try out your game.

Case Study: Pocket Frogs Cross-Promotion

All that is fine in theory. How does it work in practice? I have been using all four forms of communication for a while, and I’m definitely seeing good bumps of sales and downloads with each update and each major communication.

Last week, Ian and Dave from Nimblebit and I, decided to set up a cross promotion between Pocket Frogs and Flower Garden.


I updated the in-game news and send out an email newsletter coinciding with the latest Flower Garden update telling the players that if they downloaded Pocket Frogs from within Flower Garden, they would be awarded 5 doses of fertilizer. Nimblebit awarded players a flower if they downloaded Flower Garden Free from within Pocket Frogs.

When you have over a million downloads in a week like Pocket Frog did, that kind of player communication is the equivalent of a nuclear cannon. The effects of the cross-promotion were obvious the instant the news went live:


As you can see, Flower Garden Free made it all the way to the number 56 in the iPhone Top Apps chart in the US! The effect even spread to the paid version of Flower Garden:


Pocket Frogs at the time was hovering at around #9 on the charts, so it was difficult to have much of an impact on that position without major numbers, but we suspect it might have hovered there a little longer because of the extra downloads from Flower Garden.

Here is what the downloads for Flower Garden Free looked like for the last month. The Pocket Frogs cross-promotion is quite noticeable:


All those downloads also translated into in-app sales through the Flower Shop. Here are the revenues for that time period:


One consequence I wasn’t expecting, but in retrospect I’m not that surprised about, is that the ratings for Flower Garden Free dropped by a whole star (from 4 to 3), with a large percentage of 1-star reviews. That’s because a lot of people who wouldn’t have downloaded Flower Garden otherwise did it anyway, didn’t like it, and deleted it right away.

To wrap things up on a better note, there was yet another side effect of the cross promotion. The Pocket Frogs link was using my LinkShare referral code. Sending all those users to the App Store to download a free game link resulted in about $200 in referral profit for the week.


Communicating with your players is more than just profitable: It’s crucial to the sustained success of your games. Make sure you try to engage with them in every way you can, keep them up to date with developments in your game, and don’t hesitate to run the occasional cross-promotion, especially with other games that are a good match for your target audience.

[1] If you decide to use them and wouldn’t mind using my referral code (WQHVUF), I can get a small percentage back.

This post is part of iDevBlogADay, a group of indie iPhone development blogs featuring two posts per day. You can keep up with iDevBlogADay through the web site, RSS feed, or Twitter.

Customizable Color Sections With OpenGL ES 1.1

One of the items in my ever-growing list of things to write about, is the rendering techniques I used in Flower Garden. In the end, it would make for a post with lots of pretty pictures, but there’s nothing particularly ground-breaking. After all, it’s all limited to OpenGL ES 1.1 on the iPhone, which means only two texture units and a two-stage texture combiner. As a result, more interesting ideas keep bubbling up to the top of the list and the poor rendering idea keeps getting passed over.

Every so often, something happens that bumps up the priority of one of the items in my list. Maybe it’s another related blog post, or a game coming out with something relevant to what I wanted to write about. In this case it was a tweet from Paul Pridham [1]:


Customizing colors in a sprite or texture is very frequent in games, from changing player characters into blue and red teams, to creating color variations of an armor piece, to letting the player pick the exact shade for their pet’s fur color. Or, in the case of Flower Garden, to change the colors of the petals on the fly.

There are two requirements for this:

  • We want to change colors dynamically.
  • We only want to affect certain areas of the original texture.

That rules out creating texture variations ahead of time, although that might be a valid approach sometimes if you have lots of art resources, don’t mind increasing the download size, and you have a fixed number of variation to deal with. It also rules out modulating/blending the texture by a particular color because it would tint all the texture, and we want to limit the effect to particular areas (leave the player’s arms their normal color, but change their shirt color).

This is one of those funny cases that it was a lot easier to do many years ago, when we used palletized color modes. You could set all the custom color areas to a particular palette entry, and then update that entry on the fly. Ah, all the awesome tricks palettes opened up the door to! I still miss them to this day.

color.jpgIn modern hardware it’s also really easy to do with a shader, but Paul wanted to use it across any iPhone device, and the majority of them are still stuck on OpenGL ES 1.1, so fixed-function pipeline it is.

The simplest approach would be to just render the model twice: First pass renders the texture, and second pass renders the custom color bits (you can render them with a white texture modulated by the global color to get the right color). The main drawbacks are that you’re doubling the number of draw calls, and, with 3D objects, it gets a bit tricker because the second pass needs to use the glDepthFunc(GL_EQUAL) depth comparison function.

The better way to do this is using the texture combiners. Texture combiners allow us to perform a limited number of operations to control the final look of a pixel. We can add two textures, or multiply them, or even do a few more complex operations. The true power of the combiners is that they can be chained together, so the output from one feeds into the input of another, allowing us to create much more complex operations.

The iPhone 3G is limited two two texture combiner units [2], but even two combiners are good to create a good range of effects.

Let’s think about what we want to accomplish. We want to leave some parts of the texture completely alone and display the original pixel value. In some other parts of the texture, we want to replace the pixels there with a custom color. Actually even better, we probably want to multiply those pixels by a custom color. That way we can author the part of the texture that is going to change with grayscale details, and our color adds the tint to it.

Let’s express it mathematically. Let’s make a function M that is 1 for every pixel we want to color, and 0 for the ones where the original texture is supposed to be displayed. Our desired color is c and the texture it t. In that case, the final pixel color (p) is:

p = M*(c*t) + (1 – M)*t


We could express that with two combiners: The first one is a modulate (multiply) operation with c and t, and the second one an interpolation operation between the result of the previous combiner and the original texture, based on the function M.

Obviously M is just a mask texture. We can paint it white where we want to color the texture, and black elsewhere. We could even use the alpha channel of the original texture, but there’s one big thing to watch out for: If you have your texture as a png and process it through the default iPhone resource operations, the image will be premultiplied for you (whether you want it or not), so your color information will be set to zero everywhere that the alpha channel is zero. Oops. You’ll probably want to use the alpha channel to store transparency anyway, so we’ll keep the mask separate. If not, make sure you encode the image yourself (as raw or PVRT formats) so it’s not premultiplied ahead of time.

Are we ready transfer that formula to the texture combiners? Not quite. Apparently (and this was just trial and error, I haven’t seen it documented), the texture assigned to a combiner can only be the one at that stage. If you look at the second combiner, we would need to have the first texture as one of the parameters, in addition to the mask.

So instead, we can reorganize the function above like this:

p = c*(M*t) + (t – M*t)

What did we gain by that? The color is what’s going to change dynamically, but the mask and the texture always stay the same. We could precompute the M*t term by simply multiplying the texture and the mask. We can call that new term A. We can do the same thing with the (t – M*t) term, which just means turning black all the pixels in the texture where mask will go. That one will be B. The easiest way to “precompute” those values is just doing it in Photoshop and exporting it as a new png.


Our new formula is now:

p = c*A + B

Nice and simple! Now we can really add that to the texture combiners like this:

// c
glColor4f(m_customColor.r, m_customColor.g, m_customColor.b, 1);

// A = M*t (precomputed)
glBindTexture(GL_TEXTURE_2D, m_maskHandle);
// c*A

// B = t - M*t (precomputed)
glBindTexture(GL_TEXTURE_2D, m_textureHandle);

// c*A + B

One more thing to watch out for: Because we’re using two textures, you need to have two sets of texture coordinates. In this case, we want them to be the same, so we can just point them to the same set of data:

glTexCoordPointer(2, GL_FLOAT, sizeof(Vertex), &vertices[0].u);
glTexCoordPointer(2, GL_FLOAT, sizeof(Vertex), &vertices[0].u);

That’s it! You can see the results in the included project and play with the register combiners to achieve different operations.

At this point I was going to describe the texture combiner setup I use in Flower Garden to render the petals, but this post ended up taking longer than I had hoped for (I’m trying to shoot for an hour per post, but this has taken me already two hours between the code and the the post itself), so I’ll save that for another time.

[1] Paul developed Sword of Fargoal, by far my favorite iPhone RPG.
[2] The 3GS allows up to eight I believe.

This post is part of iDevBlogADay, a group of indie iPhone development blogs featuring two posts per day. You can keep up with iDevBlogADay through the web site, RSS feed, or Twitter.