How to construct QImage from data in std::string format by save_to_string method?

[Jian Zhu]

Dear everyone,
Recently, I begin to learn to render street map using the data from http://www.openstreetmap.org/. I find that mapnik lib is really a useful and efficient tool to render maps. You are doing an excellent work.
But I come across a problem, I use metatile to improve my rendering performance. Like the following code:

//------------------------------------------Part of my code--------------------------------------------------------------------------------------------
image_32 buf(render_size_tx*TILE_WIDTH, render_size_ty*TILE_HEIGHT); //In general, render_size_tx = render_size_ty = 8
agg_renderer ren(metaMap,buf);

ren.apply();

unsigned int xx, yy;
for(xx = 0; xx < render_size_tx; xx++)
{
for(yy = 0; yy < render_size_ty; yy++)
{
image_view vw(xx*TILE_WIDTH, yy*TILE_HEIGHT, TILE_WIDTH, TILE_HEIGHT, buf.data());
meta.setTile(xx, yy, save_to_string(vw, "png256"));

}
} 
//------------------------------------------Part of my code--------------------------------------------------------------------------------------------

Each tile in the metatile is stored in std::string format. But what I need is QImage in QT, so how can I construct QImage from the std::string data? Is it like this
"QImage img((uchar* )(const_cast(meta.getTile(xx, yy).c_str())), TILE_WIDTH, TILE_HEIGHT, QImage::Format_ARGB32);" ? But in practice, it is wrong.
Can you give me some correct examples?

Thank you for your kind consideration.
Sincerely,
Jian Zhu

[Dane Springmeyer]

One approach is to copy the image_view to a new image_32 or image_data_32 that is the same size.

And then you can call im.raw_data() to pass like the example at

https://github.com/mapnik/mapnik/blob/master/demo/viewer/mapwidget.cpp#L502-L514

For another case where we had to copy an image_view back to an newly sized image is for webp encoding:

https://github.com/mapnik/mapnik/blob/master/include/mapnik/webp_io.hpp#L94-L100

Dane

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[Jian Zhu]

Dear Dane,

Thank you for your reply.

Yes, you are exactly right. We can copy the image_view to a new image_32 or image_data_32 that is the same size. But I am new to mapnik, can you give some examples? 

And I have another question,  in the code I offered, I add the following three line, then I will get building errors in vs2010. Can you tell me why? 

uchar* mybuf = (vw.data()).getBytes();

QImage img(mybuf,256,256,QImage::Format_ARGB32);

img.save("mytest.png");

[zhan...@supermap.com]

Hi, Zhujiang 

 please post the error log ...

 I have build mapnik and the qt sample in VS2010, it seems that I can do something helpful...

 

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