Render 2d Density graph with OpenGL

[Uwe]

Store the processed data in a 2d double array "ToRenderStructObject"

• Could you please also show the definition of ToRenderStructObject and did you check how much time it takes to fill it ?
• What type of color map do you use ?
• Did you enable DEBUG_RENDER and what values do you see ?
• And are you using more than one core ?

The resolution of the data is relatively high, 1080x500 in one occasion, and 2000x2000 in another occasion, and they are painted in rather small resolution widgets-graphs since they can't both fit in the screen.
I have not re implemented pixelHint(). Is that something that would help me?

The image is rendered in the minimum of the resolutions of data/screen. But without implementing the pixelHint() the resolution of the data is not known and it will always end up in screen resolution.
Implementing a pixelHint helps, when the data resolution is below the resolution of the plot canvas ( = size in pixels ) - so in your case probably not.

My implementation of the value() function:

This method is called for every pixel - for a resolution of 1000x1000 this will be a million times and removing any pointless operation from it will have a significant effect.

• Better do these "will never happen" checks in debug mode only.
• Everything that can be calculated in advance should be done in YourRasterData::initRaster. F.e something like "(data_.size() - 1) * interval_y.maxValue() - interval_y.minValue()" is constant and does not need to be calculated again and again.
• Don't use std::round() - casting to int should be good enough.

But the implementation is not totally bad and accessing data_ might be the bottleneck.

Why don't you simply use QwtPlotMatrixData - or at least copy the code and strip it down to the minimum you need ? It is faster than your code and the main reasons for not using it ( calculate the values on the fly or returning values from the original buffer without having an extra copy ) seem to be irrelevant in your situation.

Concerning using the GPU: as long as you don't have an OpenGL canvas you would need to:

1. prepare data
2. upload data
3. create something with the GPU
4. download the result
5. translate it into an image
6. draw the image

Uwe