GUI: add CMYK painting support

This commit adds a CMYK 8bpp format to QImage. The idea is to enable the
transport of CMYK images inside Qt, for instance to be loaded/saved from
files or painted on CMYK capable paint devices (e.g. PDF).

Also, rasterization support *from* a CMYK image is added (on top of a
RGB surface), as well as CMYK image scaling/conversion.

Conversion and rasterization between CMYK and RGB isn't particularly
optimized nor it honors any colorspaces yet. The overall idea is to
match 1:1 the existing behavior of CMYK QColor (which get naively
changed to RGB; there isn't colorspace support in QPainter yet).

There are no plans to add rasterization *towards* CMYK.

Image save/load in native CMYK formats will be added in future commits.

This work has been kindly sponsored by the QGIS project
(https://qgis.org/).

[ChangeLog][QtGui] Support for 8-bit CMYK images has been added.

Change-Id: I4b024cd4c15119c669b6ddd450418a9e425587f8
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 7 insertions, 3 deletions

diff --git a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
index 15735aecf7..96f92105d1 100644
--- a/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
+++ b/tests/auto/gui/painting/qpainter/tst_qpainter.cpp
@@ -2784,7 +2784,7 @@ void tst_QPainter::monoImages()
     for (int i = 1; i < QImage::NImageFormats; ++i) {
         for (int j = 0; j < numColorPairs; ++j) {
             const QImage::Format format = QImage::Format(i);
-            if (format == QImage::Format_Indexed8)
+            if (format == QImage::Format_Indexed8 || format == QImage::Format_CMYK32)
                 continue;
 
             QImage img(2, 2, format);
@@ -3554,9 +3554,13 @@ void tst_QPainter::drawImage_data()
 
     for (int srcFormat = QImage::Format_Mono; srcFormat < QImage::NImageFormats; ++srcFormat) {
         for (int dstFormat = QImage::Format_Mono; dstFormat < QImage::NImageFormats; ++dstFormat) {
-            // Indexed8 can't be painted to, and Alpha8 can't hold a color.
-            if (dstFormat == QImage::Format_Indexed8 || dstFormat == QImage::Format_Alpha8)
+            // Indexed8 and CMYK32 can't be painted to, and Alpha8 can't hold a color.
+            if (dstFormat == QImage::Format_Indexed8 ||
+                dstFormat == QImage::Format_CMYK32 ||
+                dstFormat == QImage::Format_Alpha8) {
                 continue;
+            }
+
             for (int odd_x = 0; odd_x <= 1; ++odd_x) {
                 for (int odd_width = 0; odd_width <= 1; ++odd_width) {
                     QTest::addRow("srcFormat %d, dstFormat %d, odd x: %d, odd width: %d",