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Histogram effect

  • Článek

Use the histogram effect to generate a histogram for the input bitmap based on the specified number of bins.

The CLSID for this effect is CLSID_D2D1Histogram.

Example

Before
the image before the effect.
Graph of the histogram output data
the image after the transform. 
ComPtr<ID2D1Effect> histogramEffect;
m_d2dContext->CreateEffect(CLSID_D2D1Histogram, &histogramEffect);

histogramEffect->SetInputEffect(0, m_2DAffineTransformEffectRight.Get());
histogramEffect->SetValue(D2D1_HISTOGRAM_PROP_CHANNEL_SELECT, D2D1_CHANNEL_SELECTOR_G);

m_d2dContext->BeginDraw();
m_d2dContext->DrawImage(histogramEffect.Get());
m_d2dContext->EndDraw();

// The histogram data is only available once the effect has been 'drawn'.
int histogramBinCount;

HRESULT hr = histogramEffect->GetValue(D2D1_HISTOGRAM_PROP_NUM_BINS, &histogramBinCount);

float *histogramData = new float[histogramBinCount];
hr = histogramEffect->GetValue(D2D1_HISTOGRAM_PROP_HISTOGRAM_OUTPUT, 
                               reinterpret_cast<BYTE*>(histogramData), 
                               histogramBinCount * sizeof(float));

Effect properties

Here's the equation to generate the output.

the equation to generate the output of the histogram effect.

i is evaluated from 0 to the number of bins. The effect generates a histogram for pixel values between 0 and 1. Values outside of this range are clamped to the range. The range of a particular bucket depends on the number of buckets. This effect works on straight bitmap pixels. The color channels of the input bitmap are divided by the alpha channel to compute this effect.

Display name and index enumeration Type and default value Description
NumBins
D2D1_HISTOGRAM_PROP_NUM_BINS
 UINT32
256
 Specifies the number of bins used for the histogram. The range of intensity values that fall into a particular bucket depend on the number of specified buckets.
ChannelSelect
D2D1_HISTOGRAM_PROP_CHANNEL_SELECT
 D2D1_CHANNEL_SELECTOR
D2D1_CHANNEL_SELECTOR_R
 Specifies the channel used to generate the histogram. This effect has a single data output corresponding to the specified channel. See Channel selectors for more info.
HistogramOutput
D2D1_HISTOGRAM_PROP_HISTOGRAM_OUTPUT
 FLOAT[]
Output property only.
 The output array.

Channel selectors

Enumeration Description
D2D1_CHANNEL_SELECTOR_R The effect generates the histogram output based on the red channel.
D2D1_CHANNEL_SELECTOR_G The effect generates the histogram output based on the green channel.
D2D1_CHANNEL_SELECTOR_B The effect generates the histogram output based on the blue channel.
D2D1_CHANNEL_SELECTOR_A The effect generates the histogram output based on the alpha channel.

Data output

This effect outputs a FLOAT[], with the number of elements corresponding to the number of specified bins. Each element in the FLOAT[] is a float. The value of the element corresponds to the number of elements in that bin.

Remarks

Note

The CreateEffect method fails if the device doesn't support DirectCompute and returns HRESULT = D2DERR_INSUFFICIENT_DEVICE_CAPABILITIES. All DirectX11 cards and DirectX10 cards that support DirectCompute can use the effect.

Requirements

Requirement Value
Minimum supported client Windows 8 and Platform Update for Windows 7 [desktop apps | Windows Store apps]
Minimum supported server Windows 8 and Platform Update for Windows 7 [desktop apps | Windows Store apps]
Header d2d1effects.h
Library d2d1.lib, dxguid.lib

ID2D1Effect