Something mysterious is going on when exporting images. I'm using Mathematica 9.0.1 on Windows 7 (64 bit), and find that the memory use is excessive when exporting to a bmp. But it doesn't happen with other image formats.
For example, running the following (with different values for k
):
k = 1; extensions = {"png", "tif", "jpg", "bmp"}; SeedRandom[42];
image = Image@RandomReal[{0, 1}, {1500, 1500, 3}];
base = MaxMemoryUsed[];
Export["C:\\mem." <> extensions[[k]], image];
{extensions[[k]], (MaxMemoryUsed[] - base)/1024.^2}
Quit[]
gives
{"png", 79.987}
{"tif", 28.617}
{"jpg", 60.453}
{"bmp", 927.602}
file sizes (added by s0rce):
png: 12.8 MB
tif: 12.8 MB
jpg: 1.29 MB
bmp: 6.43 MB
Why does exporting a bmp require over 10 times the memory of other image formats? And what can I do about it?
EDIT:
Here's the results of some more testing (code at the end).
format texture type memory(MB) time(s) size(KB)
png random real 89 3.4 15 028
bit16 0 2.6 15 028
byte 0 1.5 7 515
smooth real 75 1.3 726
bit16 0 1.2 726
byte 0 0.5 85
tif random real 17 0.2 15 000
bit16 0 0.1 15 000
byte 0 0.1 7 500
smooth real 17 0.3 15 000
bit16 0 0.1 15 000
byte 0 0.2 7 500
jpg random real 68 0.4 1 500
bit16 0 0.8 1 503
byte 0 0.7 1 500
smooth real 68 0.3 50
bit16 0 0.3 50
byte 0 0.3 50
bmp random real 1069 25.3 7 500
bit16 967 22.8 7 500
byte 69 $Aborted 0
smooth real 1055 100.8 7 500
bit16 953 32.4 7 500
byte 71 1.4 7 500
Notes:
- By default Mathematica will treat the image type as Real.
- When exporting, a Real type image will need to be converted to an integer type. This requires extra memory and time.
- tif and png support 16 bit pixels, so this will be used by default for Real images.
- jpg and bmp only support up to 8 bits per pixel.
- My computer was thrashing the memory disk cache for the bmp smooth real, so it took extra time.
- The bmp random byte was using 100% for 5 minutes before I gave up.
Conclusions:
- Use tif for speed.
- Use png for saving disk space.
- Never use jpg because it is lossy.
- Never use bmp because it is broken.
Code:
k = 1; (* 1 .. 24 *)
{e, t, c} = Tuples[{Range[4], Range[3], Range[2]}][[k]];
extensions = {"png", "tif", "jpg", "bmp"};
types = {"real", "byte", "bit16"};
compress = {"random", "smooth"};
res = 1600;
SeedRandom[42];
data = Switch[compress[[c]],
"random", RandomReal[{0, 1}, {res, res, 3}],
"smooth", Table[{i, j, 1 - i j}, {i, 0., 1, 1/(res - 1)}, {j, 0., 1, 1/(res - 1)}]
];
image = Switch[types[[t]],
"real", Image@data,
"byte", Image[Round[(2^8 - 1) data], "Byte"],
"bit16", Image[Round[(2^16 - 1) data], "Bit16"]
];
filename = "C:\\_" <> types[[t]] <> "_" <> compress[[c]] <> "." <> extensions[[e]];
base = MaxMemoryUsed[];
time = AbsoluteTiming[Export[filename, image]][[1]];
CellPrint@TextCell[ToString@{
filename,(MaxMemoryUsed[] - base)/1024.^2, time, FileByteCount[filename]/1024.
}, "Text", CellAutoOverwrite -> False]
Quit[]
MaxMemoryUsed[Export["max.png", image]]
to measure how much memory an operation uses. I'm not sure how accurate it is though. $\endgroup$ – Szabolcs Mar 4 '13 at 21:35On["Packing"]
shows that exporting to BMP unpacks the whole array. No wonder it's so slow and takes so much memory. Probably it's best if you report this to support and mention the packing thing. Tagging as bug. $\endgroup$ – Szabolcs Mar 4 '13 at 21:44