H.264 All-Intra Data Rates Significantly Higher

Question

Does the built-in H.264 codec encode I-frame only files differently? I am trying to determine the optimal GOP length for high bitrate exports. Image quality seems to degrade, even in the I-frames, when using key frame distances greater than one.
After performing a series of tests to characterize the Adobe H.264 encoder, I discovered that exported files are significantly larger when key frame distance equals one frame (N=1). The average video data rate for a test file rendered with the Adobe H.264 encoder is as follows:
N=1 : 2.17 bpp : 24I
N=2 : 0.66 bpp : 12I + 12P
N=3 : 0.59 bpp : 8I + 8B + 8P
Note how the data rate drops 70% (from 2.17 to 0.66 bpp) even though 50% of the I-frames still exist when N=2. By comparison, here is the video data rate when exporting with QuickTime H.264:
N=1 : 0.89 bpp : 24I
N=2 : 0.70 bpp : 12I + 12P
N=3 : 0.64 bpp : 8I + 16P
The following chart shows data rates at key frame distances from 1-48 frames for Adobe H.264, QuickTime H.264 (via Adobe), QuickTime Pro, and Expression Encoder 4 Pro. Data rates are consistent among all encoders at GOP lengths greater than one. There is an anomoly with the Adobe H.264 codec compressing all-intra files.
The observed behavior occurs in all profiles, which were tested at Levels 4.1, 4.2, 5.0, and 5.1:
Image quality is better in the Adobe H.264 all-intraframe file, especially with respect to detail retention. The pictures below show sections of two consecutive frames magnified 400%. The file with N=2 is less accurate and contains noticeable blocking. Even the I-frames don't look as good in the files where N>1.
The test file was a seventeen second Premiere Pro sequence consisting of H.264, MPEG-2, and AE files with effects applied. Exports were rendered from the Premiere Pro timeline and from a V210 uncompressed 4:2:2 intermediate file of the sequence. The following settings were used:
Format: H.264
Width: 1280
Height: 720
Frame Rate: 24 fps
Field Order: Progressive
Aspect: Square Pixels (1.0)
TV Standard: NTSC
Profile: Baseline, Main, and High
Levels: 4.1, 4.2, 5.0, 5.1
Render at Maximum Bit Depth: Enabled
Bitrate Encoding: VBR, 2-Pass
Target Bitrate: Maximized for each Profile/Level
Maximum Bitrate: Maximized for each Profile/Level
Key Frame Distance: 1, 2, 3, 4, 5, 6, 7, 8, 12, 24, 48
Use Maximum Render Quality: Enabled
Multiplexer: MP4
Stream Capability: Standard
Software:
Adobe Media Encoder CS6 Creative Cloud v6.0.3.1 (64-bit)
Premiere Pro CS6 Creative Cloud v6.0.3
Windows 7 SP1
QuickTime Pro for Windows v7.6.9 (1680.9)
MediaInfo 0.7.62 (for GOP and data rate information)

Answer 1

I think the short answer is yes,
a h.264 encoder does I-frame only differently. I frames are complete expressions of a picture with no temporal compression information.
P frames use _P_redictive information. IE information from prior frames.
B frames use _B_i-directional predictive frame information.
h.264 gets the majority of it's bit saving from use of B and P frames. When you do I-frame only you only get the block compression and none of the advantages of P and B frames. Thus the GOP N=1 doesn't get very good bit's per pixel.
Having said all that I do find your comment...
Profitic wrote:
Note how the data rate drops 70% (from 2.17 to 0.66 bpp) even though 50% of the I-frames still exist when N=2. By comparison, here is the video data rate when exporting with QuickTime H.264:
... very interesting. Indeed, why is the datarate 70% less when it should at best be 50% for GOP N=2. 50% less should be the same I-frame information plus 0 bytes for the B frame between them. (GOP = I,B,I). Any more than that and it is throwing away bits from the I-frame. So, this seems to be to be a ratecontrol bug.
http://en.wikipedia.org/wiki/Group_of_pictures
"The GOP structure is often referred by two numbers, for example, M=3, N=12. The first number tells the distance between two anchor frames (I or P): it is the GOP size. The second one tells the distance between two full images (I-frames): it is the GOP length. For the example M=3, N=12, the GOP structure is IBBPBBPBBPBBI. Instead of the M parameter the maximal count of B-frames between two consecutive anchor frames can be used."